Brake bias issues
 

I have an SA with the following brake set-up:

Front: Lightweight aluminum hubs from KC Raceware, 11 inch rotors from Coleman Racing and FD calipers.

Rear: Lightweight hubs from KC Raceware, and GSL-SE calipers.

Original master cylinder and booster AND original proportioning valve.

Hawk HP pads all around

When I installed this set-up it seemed to be fairly well balanced front to rear, at the time I was on not so sticky street tires (Yokohama S-Drives).

Since then I have started tracking the car more, and have upgraded to Yokahama AD08R tires.

I am becoming aware that I may have too much rear bias for the track. The symptoms are occasional tail happiness under hard braking, especially if I jump on the brakes very suddenly, brake hop under certain situations, especially if I soften the rear shocks, (on one occasion this was sever enough to bend my rear axle housing!!) and accelerated rear tire wear.

I'm looking for suggestions for how to best deal with the situation.

I thought of adding an aftermarket proportioning valve, but my research has revealed that since my car was originally a rear drum car then it's original prop valve includes a "residual valve" to hold a small amount of pressure to the rear brakes, so I'm thinking I would like to get rid of it. Been trying to figure out a plan to do this.

I have researched and found that the FD caliper bores and the GSL-SE bores are the same, so couldn't I just install a proportioning valve from a FD and have the bias I need?

Thoughts?

One other thing to look at is the stance of the car. If the front is lower than the rear it will increase weight transfer and cause similar symptoms. I had this happen on my 2nd gen E Prod car and leveling it cured the problem. Installing a manual prop valve and eliminating the factory valve would be advisable also.

That is an interesting suggestion.

I recently had the car corner balanced and the shop raised the rear slightly, and now that I think of it I did notice more nosedive under hard braking.

Well, the further I research, the more complicated it becomes.

I have researched and have found that it is definetely not reccomended to add an aftermarket adjustable proportioning valve downstream from the factory proportion / bypass valve. It seems that installing them in series can cause "very nasty" things to happen, so I am leaning toward substituting a factory valve in its place. I just have to pick between a later 1st gen valve from a rear disc brake car, or maybe a 2nd, or third gen valve. So I am trying to decide what the criteria should be, if it is caliper bore size and disc diameter, then the 2nd or 3rd gen valve seems to fit best,

One thing that has me confused is that all information I can find suggest that the proportion valve I have (from a drum rear brake car) SHOULD reduce the rear pressure more than one for a disc rear system, doesent make sense that I am having too much rear braking.

Maybe I should hold off on any changes until I experiment with lowering the rear ride height to what it was before...

I apologize if I am thinking out loud too much.............

By changing the rotor sizes you have changed the brake bias. Why don't you just remove the OE prop valve and replace it with a Wilwood unit?

You may also need to try a less aggressive pad. That is what I had to do in my FC.

I have bigger front rotors than OE and stock rears. I ran DTC60 pads all around and had to switch to DTC30s in the back, and adjusted the prop vale until the bias was where I wanted it.

since changing the ride height is the easiest thing, start there.

next step is either an adjustable prop valve, or less aggressive rear pads, or more aggressive front pads.

I will adjust the ride height to see if that helps, but I am still trying to understand all the issues involved.

I don't want to go to the aftermarket adjustable prop valve, since to do that I would need to remove the factory valve and by doing that I would loose the bypass valve, which is a big safety factor,( it shifts the flow to the rear brakes if the front looses pressure, I think, allowing you to have at least some brakes).

I know that changing rotor diameters will change the bias, but all the changes I made, (larger front rotor), should have shifted the bias to the front, not to the rear, and the drum brake rear proportioning valve should also cause too much bias in the front, not the rear, so I still don't quite understand why it is the other way around, and, since I don't fully understand the problem I will not change the valve yet. I will try to address the issue with j9fd3s suggestion and put more aggressive pads on the front, if the stance change does not fix the issue, that is.

The car has a dual circuit master. Both circuits are isolated within the master cylinder. In the event of a pressure loss on one end of the car, the other will still work.

I don't think messing with the stance of the car is going to fix your problem.

The residual valve is there to counteract the springs in the drum brakes, not protect you in the event of brake failure. The drum brake prop and residual valves are most if not all of your problem. You don't need any residual valve with disk brakes generally speaking.

Keep in mind the prop valve pressure output is not linear. There is a knee point in the buildup of pressure. I'm not a brake engineer, but I'm almost positive the knee point needed for disks and drums to work is different.

Do the rear brakes lock up every time the car gets tail happy at corner entrance?

Yes,
or put another way the only time it gets tail happy is when they lock up. The car is very well balanced, well enough that I can get it to turn in more mid corner by gently manipulating the throttle. When it is near the limit I can steer it with the throttle, the only tail happiness is when I a have to jump on the brakes too suddenly like when I'm in too hot. If I am not abrupt with the brake peddle I can still get fairly good braking without rear lockup of brake hop, but I do have to be gentle with the original application of the brake. That having been said I am still convinced the bias is off because the last track day totally wore out my rear tires, WAY more than the front.

I understand that the residual valve holds pressure to the rears, but it also has a bypass valve built in to redirect the front fluid flow in case of a pressure loss on the front, maintaining braking in case of a front brake line failure, I don't want to loose that safety feature.

I agree that the knee point and slope of the valve is probably way different on a drum vs disc car. That, and wanting to keep the byass valve is what has me considering using a 2nd or 3rd gen combination valve.( All the factory valves are combination valves that include a bypass valve). The caliper bore surface areas are exactly the same as what I am running, and the rotor diameters are very close. It would have to be closer than the stock drum valve that I am currently running.

I am running a track day later this month at Atlanta Motorsport Park and will adjust the stance just to see if that helps, if it does not I will likely change the valve to a 2nd or 3rd gen and see what that does. I may also change the master to a FB unit from a rear disc car. I cannot ascertain if the drum master cylinder I am running is inappropriate for a disc / disc car. I have tried to find technical drawings of it to see if it is different or not. Cannot find one.

At this point I am open to suggestions, thanks for any help.....

You have a safety system in the dual circuit master cylinder that is in the car. If you lost front pressure you still have rear brakes. That is why the reservoir is divided.

I don't believe the later prop valves in the FC have any bleed back facility. I believe the front line is just a pass through on the valve.

If you don't mind sharing, what kind of shocks and struts are you using and what are your spring rates. I ask because an issue like this can be caused by too much weight transfer at corner entry.

What I have found is that when a race car is properly sprung and has good shocks/struts you can run more rear brake line pressure than what a OE prop valve will provide.

I am not totally discounting a brake bias issue but my guess is that this isn't your only issue.

I have RE- Speed coilovers on the front and the adjustable spring seats on the rear with Tokico Illuminas adjustable shocks all around. I also have the RE Speed rack conversion and the RE speed front sway bar (really big) . The spring rates are 200 on the front and 175 (or maybe even 150)? on the rear. I know this is rather soft but I choose these rates to avoid the bone jarring ride that my old stiffer springs gave me. You think this is contributing to my problem? If so, what can I do to minimize this problem short of going to higher rates?

I liked the illumines when I had them on my racecar. I even have the modern version on my 2012 Mustang!


Those shocks and struts adjust rebound primarily but compression damping does seem to go up when rebound stiffness is increased. On the rear I would keep them set at 4 or 5 all the time because you don't want the rear end coming up under braking. In front I would keep them at 3 or higher depending on how the car feels.


You mentioned that the car sits higher in the rear now. As j9fd3s suggested, you might want to lower it back to where it was. This will shift the center of gravity towards the rear.


Your spring rates are pretty soft which is something that sticky tires will magnify. For comparison my IT car had 400-450lb springs on the front and 200-300lb springs on the rear. I don't think you have to go to those rates but going up 50-100lbs in the front should be doable without allot of impact on the car's ride.


Just as you want a racecar/performance car to stay flat during corning, you also don't want it to nose dive under braking.


Regarding the prop valve and brake bias with the stock master cylinder - we have a couple of E Production 1st gens in the area that still use power brakes. They seem to work fine. Both of these cars started life as either a GSL or GSL/SE so they had rear disk brakes. The point is that OE prop valve from a disk brake car may be something you need to try.

Thanks for the input!

Here is my plan:

1. Lower the rear as suggested

2. Be sure to keep the rear shocks firm. I know this has had an affect because when the brake hop was the worst (and I bent the rear axle housing) I had softened them just to see how it would affect the corning balance of the car, big mistake! Your insight verifies this.

3. My pads are due to be replaced, so I am ordering some that will shift the bite more to the front, I will probably use the Hawk HPS on the rear and the HP Plus on the front. Thoughts?

4. Replace the master Cylinder and prop valve with a disk one from a FB.

5. Re-evaluate and possibly raise the front spring rate.

Thanks for your help!

the above is a good plan. if hawk makes DTC pads for a 1st gen you should look into those. they are much better, although still about 10 years out of date

Sounds like a good plan!

I looked at the DTC pads but was concerned that there higher heat range would make them inappropriate for the street. The stated range is 400-1600 f. I have no experience, so I was concerned about long stops when cold. I dont want to rear end grand ma :).

Thoughts?

dunno, we have a dedicated race car, so no street use. we haven't used the hp/hps stuff in 10+ years, and literally just went through the first set of DTCs so its a bit early to say. generally newer pads have fewer tradeoffs than older ones.

for example, we have run PFC pads for years, and they are almost miraculous. they modulate very well, which the drivers really like. they don't fade. pad life is really good (so far about 4x the DTC's), they are really easy on the rotors, and don't even dust much (much less than the DTC's in fact)

If I can add a couple of things to the conversation. First, I used a band aid on my FC to prevent premature rear brake lock up that has served me well for years. I use Hawk Blues on the front and a standard Auto parts store rear pad. On the warm up lap, the rears heat up quickly and work immediately so you need to be a bit careful. After I start racing, the rears heat saturate pretty quickly and the fronts take over. For some reason it works extremely well on my car and gives me great modulation and predictability.

Other concern is that if you are starting to look at the DTC and their ilk, they are great pads. The only downside is their heat range and how you really need to be in it for them to work. They are not kidding when they say 400 to 1600 F. If you are below 400 degrees constantly, they will chew up brake rotors like they were made of rough granite. Be honest and take brake temp readings. Then choose a comp pad that works for your peak and regular operating temperature. It may not be what the fast guys are using.

Eric

What compound PFC pads did you run? And what compound DTCs are you running? I'm running DTC 60/DTC 30 on my RX7. I like them and they actually work on the street without feeling like you are going to die. My understanding is DTC70s won't work on the street as you won't get them hot enough. The 60/30 setup is easy on rotors and cheap, but I go through them quickly... About twice as fast as 100 tread ware tires.

i think the last set of PFC's was 18?

and BTW the drivers did not like the same compound all the way around, it locks the rear too fast, and its a miata. so we were running 18 in the front and whatever was handy in the back.

i didn't get to see what the DTC's were, i think 60's? i get to put the next set on in a couple weeks, so i'll look. we got one race weekend, 2 track days, and half way through the Miata's at laguna seca event*, before the DTC's were halfway through the backing plates! this is WAY shorter than the PFC's, we get 3-4 race weekends out of those**

*we had to change pads in the pits after the parade lap!
**depends on the track, laguna is hard on everything, thunderhill is really easy

Progress report:

Installed new pads, Hawk HP Plus on the front, kept the HPS pads on the rear, and a new set of Ado8r tires, lowered the rear of the car about 1/2 inch and adjusted the shocks to the firmest setting.

I went for a drive through the country to bed the pads in, carefully following the instructions on the box and the results are very good.

I am very impressed with the additional bite of the Plus pads over the old HPS pads I had before.
Big difference!!!!

The old pads took way more peddle pressure, more so than I would have ever guessed.

This seems to have fixed my issue, I guess I will have to wait till the next track day to be sure as I did not want to go full race mode on the roads around where I live, even if it is way out in the country.

I still don't know how they will act when everything is good and hot, but so far, so good, the fronts are locking up first every time now. I am hoping that it is not a problem that changes as the tires begin to wear as today's experiment was done on brand new tires. We will see.

Hawk HPS pads are horrible.. Low bite, mushy feel, fade easily. As you noticed HP+ are great street/light track pads. The initial bite and feel is much much better.

qft

HPS pads cant even handle spirited street driving

Well, I don't think the OP will have to worry about premature brake lock up now. After about 3 laps, the front pads will be heat saturated and stop working. The rears should be as well by then. I don't know of anyone who has used the HPS or HP+ pads on track during a dry race and had good results. If you find they are working for you, I would suggest you aren't braking hard enough.

A decent race setup on my buddy's FB was Hawk Blues on the front and Blacks on the rear. He ducted to the front and rears and they worked well and lived a decent amount of time. He never complained about premature lock up.

Eric

Keep in mind this is a street driven car used for Track days and not full blown races. Twenty minute sessions on street tires would have to be generate less heat than a bonifide race car on slicks used for full length races.

Given, now that I've tried the HP + I would agree that the HPS pads have no place on the track, I will see how the HP + holds up next week end at AMP, if not I will step up once more till I find something that works for me and just switch over for track days then back again for the street.

Update:
 

Well after driving the car some I was able to come to the following conclusions:

Firming up the shocks and lowering the rear have definitely helped, the brake hop has disappeared, even when the rears lock up first. Thank you mustanghammer.

And, installing the more aggressive pads just on the front fixed the bias issues, but like most systems, when you change one thing it affects others. In this case it made the brake peddle too sensitive. Before the peddle was very firm, which made it easier to heal toe, now when I heal toe the it causes enough variation of brake peddle pressure that the braking varies uncontrollably, makes controlling the braking impossible. So I have reinstalled the old pads until I can come upon a proper fix.

I think this means I need to install a master with a larger bore to firm up the peddle so the braking is more controllable when heal towing, so that is the long term plan, switch to a later year model master with the larger piston, and possibly an
aftermarket proportioning valve as suggested by Large Orange Font, although I am still curious about the function of the bypass valve in the stock prop valve

I have a track day this weekend and don't have the time for a proper fix so I am considering a band aid. I am thinking of reducing the area of the rear pads by trimming them back, just to get me through this last track day of the season, then going back during the off season and fixing the system properly.

What do you guys think? Stupid Idea?

it sorta works. we've got a bunch of stuff like that in a box somewhere, theres a prop valve that has the big bolt replaced with a nut and bolt to make it adjustable, stuff like that.

plan B would be to drive it a little differently. maybe brake a little earlier, and less "jumping" on the pedal. it is possible, you could brake earlier, straighter, and then be braking less at turn in. one of the easy data logs to interpret is braking. our fast drivers brake pressure trace is basically square, high and short, and then there is a pause and then the accelerator goes WOT. so he brakes as hard and fast as possible, but he's also braking in a really short amount of time, then there is a release, and he doesn't hit the gas until it can be matted. compared to our slower driver the faster one brakes sooner, and his mid corner speed is the same or slower, but he's faster because he spent less time braking, and then he goes WOT faster, and for longer

oh and just to be pedantic, its a brake pedal. peddle is like; to sell.

Good information, Thanks for the input, I will probably take plan B.

PS.... Do you really think a guy that cannot spell pedal will know what pedantic means? LOL!!!

I would do plan B as well. As you go to more aggressive pads, the braking characteristics of the car are going to change, especially when the pads are at operating temperature. Just try to be smoother with the brakes on entry and complete all of your braking before you begin turning. Keep your line through the corner neat and get back to the gas as soon as possible.

The HPS pads were so crappy you could get away with slamming on the pedal anywhere because they had no feel or bite. When you jump on the brakes with HP+ pads you get a ton of bite and feel which you are not used to.

I am glad you are headed in the right direction, but still get a full disc brake MC and prop valve setup on the car. A larger MC will help you. I put a larger 929 master and booster on my FC and am very happy with the change.

we've tried a few different pads over the years, and the drivers go for feel/modulation over just about everything else.

Glad the shock settings worked for.


While I have had street brake pads that felt spongy this is not a PSI issue. The newer pads have better initial bite on the rotor.


It really seems like the new pads are providing improved braking. Once you get used to them and learn how to utilize the improvement, you'll be faster.


Peddle....usually I see "breaks" and have to shake my head!


Keep us posted on your results

That and tire "thread".

My track day at AMP was good. Plan B worked well. As long as I was smooth with the original application of the brake I could still end up standing on the pedal hard without total rear lock up and by the end of the day could even turn in with a little brake, creating a momentary and subtle oversteer that I could control into the apex and then accelerate from there. I had a very good day and ran down some very fast cars, granted with slow drivers, but fun to do anyway!!

The only down side is that I could tell some other cars were out braking me, letting me know that my setup was not getting enough braking out of the fronts.

Any way I am considering what direction to go for the final remedy.

I sought out the local brake expert at the track who happen to be in his 240Z race car with brake upgrades that were similar to mine. He is an engineer and had tried many various setups to give the right balance.

He is steering me in the direction of double masters with a balance bar. I am considering this as other solutions seem to me to be experiments, and the double masters, with a little tweaking of master bore should work very well, with less guess work.

My concern is how such a setup would work for the street, can I find a pad that will do double duty? Some of these newer compounds you guys have experience with makes it sound doable.

What do you guys think?

and btw, thanks for your help!!! I have driven this car for years and done a lot of autocrossing, but the track thing is relative new to me, been doing it about a year and a half now and am just now getting to the point that I am not totally ham fisted, and with the higher level of driving I am uncovering issues, such as these brakes, and I am having a ball, not only with the driving but with engineering solutions for the problems, so once again THANKS!!!

Now off to figure a fix for fuel starvation under braking for Weber 36 DCDs!

JGrewe has posted the dual master sizes...

I hear what you are saying and after doing this for a few years I would like to ask you a few questions about why you think other guys were out braking you. Were they on full throttle longer and then hammering the brakes and getting stopped? Were you locking the fronts? Were you pushing as hard as you could and the car wasn't stopping any faster? Were you locking an inside front? Did you melt out your front brake seals?

The reason I am asking is that there are so many other things to do to maximize your braking performance before you throw parts at a car. It may be my history of coming from showroom stock and World Challenge style racing, but the stock brake setup can be made to work extremely well and usually is only a problem when you exceed the heat shedding capabilities of the rotor, hub and calipers. If you are not melting seals, then you aren't exceeding the thermal capacity of the setup. That means the rest comes down to setup. Setup includes Front to Rear brake bias, pressures, alignment settings and tire size and grip. Sometimes if you are tire limited then you need to adjust the camber to maintain a flatter front tire contact patch and sacrifice some potential cornering grip for braking grip. If you aren't, then go grippier or wider until you exceed the braking capability of the front caliper and rotors. Also, In my opinion, unless you have huge front rotors and large calipers, there is no such thing as a dual duty brake pad. If you are able to drive around all day long with a under 12" front rotor and a pad that works on the street, then you aren't going hard enough. If you are exceeding the heat shedding capabilities of your front rotors or fading the pads (exceeding their capabilities) then you need to go to a higher heat stable material and more ducting. Only after you grossly exceed the capabilities of the stock setup, should you look at upgrading.

My race RX7 has won many races, has finished first in class in the last 2 years 3 Hour Enduro's at Mosport, set the overall 3rd fastest race lap in the 2015 race, runs on slicks and 15x7 inch rims and still uses the stock single piston, single master, non vented rotor setup from a standard 1988 RX7. The brakes work great and with careful pad management, the brakes last hours with no issues. I am just trying to say that sometimes if you think intelligently about the situation and adjust your driving a bit, your setup a bit and watch the wear, you don't need to spend cubic dollars on rebuilding something that works absolutely fine.

Eric

That's where I am now, trying to evaluate my situation so I can go in the right direction. Don't want to go off half cocked and waste time and money.

To your question about getting out braked it was from watching cars close in on me at the end of the longest straight. Prior to the braking zone I would have four or five car lengths on them that would go away. They were braking later and harder than I was. Keep in mind that I was trying to be smooth because I definitely have a brake bias problem and if I brake harder the rears lockup. More evidence came when I examined the video I took with my new iphone app that gives g force information. The braking g s appeared to be modest to me. The reading were in the .5 range. I not sure but I think I should be able to have much greater deceleration than that. I had to be careful not to get too much rear lock up. The last time it happened I was in way too hot and had no choice but to brake as hard as I could and I got severe brake hop that bent my rear end housing. I also was concerned about losing the rear end and trying out the gravel pit, so I was intentionally braking a little early and trying to be smooth. I eventually got to where I was consistently able to brake with the rears beginning to lock up but I was still not getting the braking I think the car is capable of. Also I spoke to the other drivers who told me about how they were braking much later than I.

So the whole issue to me is fixing my bias problem. It is of my own making cause I upgraded my brakes to a non stock setup so Im having to come up with a master / prop valve combination that will give me a better balance. I had no choice but to upgrade somehow has the stock SA setup was so bad that It would never handle anything beyond modest auto cross use. Here is a link to my brake setup:

https://www.rx7club.com/1st-generati...twist-1029145/

The bore sizes on this setup are identical to a turbo II setup and the 3rd gen brakes (I think) so Im thinking I can use a proportioning valve from a turbo II car , or go with the dual master setup.

I don't enjoying wasting money, but if I have to spend some to get this right I will.

Why don't you just add an in car adjustable brake bias valve? This has been used on many many race cars and street cars to adjust the rear brake pressure to a point that make sense to you. You can do it in car while driving as the point that you need to pick will change depending on temps, tire types and weather. I have them on both cars and use it a fair bit. My Cougar has the balance bar set up and 2 masters and I adjust the balance bar to get the better braking effect, but I use the adjustable bias valve to set the max braking effort to the rear brake circuit. Think of it as a manual bias valve. Good ones are just over $100, easy to plumb in and you can play with it from where you are right now to a point where you like the rear braking effect.

After you install it into the rear pressure line, it splices in, you can set it at wide open then slowly restrict the flow to the rear until you get to a point that works for you on that day. Its not rocket science at all and most race cars have them as you need something that can adjust to the different rear loading as the fuel runs out. I use it and the balance bar, all the time in my Cougar to effect maximum braking effect as the fuel and track conditions change in longer races.

If you look at the attached picture, the blue knob is the balance adjuster, there is a small knob between the blue adjuster and the seat where I can adjust the pressure. The black gated knob is for the in-car adjustable bars that the Cougar has.

Eric

Oh and yes, I was one of the 7 people who purchased the Platinum coloured harness that Racequip were selling.... I thought that they would look cool and not LAVENDER!!!!!!!

Eric

Put an adjustable valve on instead of another prop valve.. Your problems will be a knob turn away. I will say it again.. you are not losing any safety features that way, the master cylinder is dual circuit and the front and back are plumbed independently.

I left my prop valve under the hood for a simpler setup.

https://cimg2.ibsrv.net/gimg/www.rx7...673e1e6aea.jpg

You know what, I'm going to do just that. I have thought about it till my head hurt and I have decided to rely on your experience. The only additional change I shall make is going to a larger bore master cylinder so I can have a firmer pedal with pads that have more bite than the wimpy street pads that I have ran in the past. I will have to decide on which of the newer pads to install that will have better bite than the HPS pads I have now, and are not as rough on the rotors as the HP Plus pads that I have tried recently.

Do you have a recommendation on a brand for the proportioning valve?

I used one from Stainless Steel Brakes as that's the one I could get quickly. Any of the name brand ones are fine. They all basically work the same and are all reliable. Remember to keep your stock proportioning valve in place. And without be laboring the point, why are you changing the master? If you increase its size it will make everything more sensation to pedal pressure . Be ready to have to learn to modulate the pedal even more than now. I am not saying to not do it, just saying that you need to understand what happens when you do something.

Eric

My research says, and my understanding is that a larger bore master will give a firmer pedal with less absolute braking capability and a smaller bore will give a less firm pedal with more absolute braking capability. Braking capability meaning more PSI in the hydraulics.

The master in it now is the stock one, which is the smallest bore ever on a Rx-7, and also it was originally a drum brake car so I'd like to get rid of it and get one that will yield a firmer pedal.

very nice setup!

Absolutely do the larger master, just be aware of the port configuration. On the 929 MS I used, it only had one fort for the front brakes, so I had to get the dual banjo fitting you see in the picture. The FC master cylinders have 2 front ports.

I would do the Flyin Miata prop valve kit (it is what I used). It comes with the generic Wilwood prop valve and the standard to metric conversion fittings. I used premade metric brake lines from the auto parts store to replumb from the master to the valve, and that was it.

https://www.flyinmiata.com/complete-...oning-kit.html

The cable mount is nice, but I honestly don't mess with mine at all after getting the braking dialed in. Conditions are pretty consistent for what I do with the car.

Thanks!!!

I will do research to try to guesstimate what diameter bore to try first.

My heal toe style is probably unorthodox as I brake with my heal (read stand on the pedal with all I've got) and throttle with my toe and it all works best with a very firm pedal. I've been doing it this way for so many years that I shant try to change now, so I will probably go with as large a bore as I can find to start, if I have to trial and error the size I will. I have found willwood prop valves that come with 10 x 1 mm threads eliminating the need for adapters AND they even have one configured to split the front line so I can use a two port MC if I need to without adapters, giving a neat install hopefully.

I am more concerned with the mounting lugs. It will be easier if I can find a large bore MC with lugs at 12 and 6 rather than 6 and 9. If I can it may bolt right onto my booster, easy peasy. I remember seeing some threads about the different possibilities, I will search. The largest MC I have found so far with vertical lugs has a 7/8 in bore, do you guys think this is big enough? All the FC cylinders are larger (maybe 15/16 ?) but have horizontal lugs that will not bolt up. I will continue to search.

You may want to change the booster along with the MC if feasible. I do not know what the FB booster mounting pattern looks like. If it is the same as the newer Mazdas, this gets very easy. When I swapped mine I put in a 929 booster as well as the MC. The 929 MC is 1 inch. The FC, 929, FD all have the same mounting setup, as well as many others.

Subaru has some 1 1/16 MCs that bolt right on to the Mazda boosters with horizontal mounts.

Clockker on this forum has posted tons of information about the MC and booster swaps.