FC 4-Pot Brake Conversion, Soft Pedal
 

I made this conversion on my '84 GS, and the brakes are somewhat mushy. I checked with another member, and he experienced the same thing with his conversion. Here are the parts and pieces:

- Teflon brake lines, front and rear
- '84 FB 12A master cylinder and prop valve
- FC 4-piston calipers and rotors (re-drilled for 4 lugs)
- FC 4-lug hubs
- FC to FB conversion brackets and hub adapters

I've bled them numerous times. The pedal is firm initially, but gradually softens with use.

Have others experienced this? Solutions?

Bleed the master. Over and over. Granted, mine was on a stock system- but I was having the same issue after swapping to braided steel brake lines. Finally decided to try bleeding the master, and my pedal firmed right up.

The other thing, that master may not be moving enough fluid for the extra 2 front pistons.

You may need an FC master cylinder and booster. I've never done the swap honestly. I know a buddy of mine has though. His username is 82transam. May want to shoot him a pm and see if he had similar issues and what FC parts he used with the swap.

the FC's in general have a mushy pedal, or rather travel is longer than we're used to, they do tend to be pretty linear though.

i don't think this applies to you, but on the FC the clearance between the pushrod on the booster, and the master cylinder is important, spec is 0 or nearly zero, and for example my FC had like 0.060".

What is the orientation of the caliper? The direction, up/down, of the bleeder matters. Also, did you start with a rear drum car? Not sure if the proportioning block plays a role or not.

I think it could be the bleed or prop valve. My 85 GSL has FC 4pot front brakes and the pedal is incredibly firm and short

Mine seems to always be consistent. I am using the FB booster/master/prop valve.

What rear brakes are you using? If you're also using FC rear calipers they need to be flipped as the bleeder will be in the wrong spot and never allow a complete bleed.

How old is the master cylinder? An inconsistent/spongy pedal, despite being fully bled can point to failing seals inside the master cylinder... I've had that happen on a few cars - never fun lol

Note from my first post that the pedal is very firm initially, but softens as I drive it.
- I'm sure there aren't any leaks; I've inspected for it, and the fluid level doesn't drop
- Rear brakes are GSL-SE
- All bleeder screws point upward

I experienced pad knock-back on my other car. This is caused when there's enough relative movement between the rotor and the caliper that it knocks the cylinders back into the caliper. When you apply the brake, you have to push the pads further before they contact the rotors, which gives dead travel before they feel firm.

I think a simple test would be to see if the problem is worse after hard cornering vs straight line driving.

I checked the free-play in the hubs, and they seem to be tight.

Subscribing to this thread to see what comes of it. My setup seems to have somewhat spongey of a pedal as well even though I went from GS to GSL-SE brake setup with stainless lines - much less of a jump.

A friend of mine locally did the swap, and mine is complete, but not running... We both used a 1 1/4" Wilwood master cylinder. It bolts directly to the 2nd gen NA booster (not the Turbo. That's a different MC pattern). Apparently they feel dandy!
And it's a rather large MC

Firstly, not all FCs are created equal

Series 4 FCs turbo and non-turbo came with a master cylinder with 22.22mm / 0.875" (7/8") bore.
For the series 5 onwards (including all FD), Mazda increases this to 23.81 / 0.937" (15/16") bore for the 4 piston caliper models. Presumably this was done to improve pedal feel.

When people say the pedal feel is soft on a FC, I wager this was probably on a series 4 model.

Now, for the North American FB 84-85 the technical data and service manual says there was only one master cylinder type, which 20.64mm bore (0.81inch) (13/16"). This is significantly less than even the 86-88 FC models and would result in poor pedal feel when used with the much greater displacement 4 piston calipers.

In the factory technical data and service manual, the same bore size master cylinder is specified whether the car was equipped with rear discs or not. And yet, in the parts catalogue there are actually 2 different master cylinders listed depending on the car had rear vented discs or drums.

All Australian delivered series 3 RX-7s (SA22C, 84-85) came with vented rear discs, 14" inch wheels and 4x114.3 stud pattern standard. It came equipped with a 7/8" (0.875") bore master cylinder like the 86-88 FC.

Despite what the technical data/service manual says, I reckon the GSL/GSL-SE master cylinder is probably the same bigger piston bore as my Australian one. The part numbers are the same. Perhaps someone can confirm - the bore size should be cast into the body of the master cylinder itself - mine says 7/8 on it.

The OP has a GS, meaning it almost definitely has the smaller bore master cylinder. This would explain the shitty pedal feel.

If I'm right about the GSL/GSL-SE master being bigger diameter, it should offer an improvement and will bolt straight on.

Even better would be an upgrade to a 15/16" or 1" master. Mazda 929 and Nissan Patrol are both popular upgrade options in the 1" size from memory.

the US S4 and S5 FC's have the same masters, except the S5 T2.

the US S5 T2 and 929 use a completely different master cylinder mating flange, and aren't compatible with the earlier stuff.

I'm chiming in to subscribe as I plan on doing this mod in the future.

While i can't say I know much about the swap and the details of if, I can speak to general brake diagnosis. If your pedal is firm when the brakes are cold and then begins to feel spongy as they warm up, it sound like air in the lines somewhere or possibly even a brake drag that's causing the brake fluid to overheat. I recommend to take a couple mile road test and then jack the wheels up and make sure the wheels spin freely. If they don't, then inspect for cause of dragging brakes (calipers, pushrods or brake pedal adjustment).

If they spin freely, then bleed the brakes by taking a vinal hose, submerge it in a clean bottle of brake fluid, stick the other end on the bleeder, open the line and pump the brakes until all the air comes out. Just be sure to keep the reservoir full of fluid. And be sure to use new fluid. Brake fluid can go bad quickly if left open to the elements. Good luck.

It looks like the 1992 - 1995 Mazda 929 MC (1" Bore) has brake line fittings in the correct locations, which will save bending new lines. The interface to the booster, however, is different -- it has a tube extending outward, whereas my current MC has a hole.

Does anybody have a booster suggestion?

Apologies for the confusion. The 929 upgrade is for FD RX-7s. I had my wires crossed. That said, for RHD vehicles at least, it is not a difficult feat to bolt in an FD brake booster, giving the 15/16" FD master cylinder and option to upgrade to the 1" 929 item.

For a standard 1st gen brake booster, the upgrade options available are:

1. GSL-SE brake master with 7/8" bore - this should offer a slight improvement over lower spec 13/16" options. It is also brings it up to the same spec as Mazda had on 86-88 FC models.

2. a master cylinder from a Nissan Patrol G60 (1973-1980). Now from my research, this car was never available in the US, but master cylinders for it are available online. In Australia, the aftermarket brake manufacturer ProTex makes a version with part no. JB1419. It is a tandem master cylinder with 1" bore, but uses twin reservoirs - 1 for each chamber. The bolt pattern is identical to 1st gen RX-7 and the brake lines are on the correct side. Some minor mods are needed but they are not difficult, see here: brakes

Here's some details on fitting an FD brake booster and master cylinder to 84-85 model. It's for RHD, but might still apply, as the master cylinders were the same for RHD vs LHD.

See: AusRotary.com ? View topic - S3 with S5 rack and pinion, suspension and steering...again!

No apology necessary. I appreciate the info!

That Nissan Patrol MC looks interesting, but it will require some brake line work, since it appears there is only one outlet front and rear.

It would be an easier overall upgrade if there is some booster that would fit next to our LHD clutch MC and mate with the 929 MC.

People use a simple T-piece for the outlets on the Patrol MC.

FD RX-7. It clears the clutch master cylinder for RHD - for series 3 at least. On RHD models, they actually moved the clutch master cylinder for 84-85 models to provide more clearance as the brake booster was bigger.

Hopefully the same for LHD - but obviously spacing may differ. I imagine if it could be done, someone would have documented it on this forum.

the brake booster to firewall pattern appears like it is an SAE standard or something.

every 78+ Mazda uses the same pattern, as does Honda, Subaru, etc. the are subaru's with masters larger than an inch as well.

and the real clue that its a standard size is that even Ford uses it, and they don't share anything with anyone

The s4 brake booster will fit. You need to oblong the holes a little because the booster will hit the clutch master. Trochoid did this on his wide body.

It's not in his build thread. But I have seen it in person. https://www.rx7club.com/1st-generati...update-575526/

I checked my other FB that has a big brake setup that I designed, and that car has the 7/8" bore MC, along with 2lb residual pressure valves on all three brake lines coming out of the MC (2 front + 1 rear). The "dead zone", firmness, and linearity are good on that car.

I have learned over that past few days that many newer cars have residual pressure valves in the system to reduce the "dead zone" before the pedal builds pressure.

I see three viable options:

1. 7/8" bore MC for FB (same as above), with the addition of RPVs. This is a good system on my other FB, so I'm confident in this setup. All bolt-on.

2. 1" bore MC for Nissan Patrol, with the addition of RPVs. This is likely to provide an extremely firm pedal, which I like. It will bolt directly to the FB booster, but will require brake line work. In addition to not having enough outlets, the front outlet is in the front, which is the opposite of the FB setup.

3. 1" bore Mazda 929 MC with RPVs and an appropriate booster. It should also provide an extremely firm pedal. I'm still searching for a booster that has the relatively small 8.6" diameter that will fit next to our clutch MC and has the correct interface to the 929 MC. This option is ideal because it won't require brake line work.

That is North American GSL-SE brakes. You could get drum brake 4x110 RX-7s here all the way up until 1985...

The interesting thing is that, at least for US market, all single piston front calipers have the same bore diameter and ALL RX-7s with rear calipers (including FD) have the same bore diameter. The only difference between GSL-SE and GSL rear calipers is to accomodate the wider rotor, they are internally identical.

Interestingly the '80-85 12A and FC calipers are mechanically interchangable if you use the correct hose and lower slide pin. My '84 (the one with the FC frontend) has an FB caliper on the left and an FC on the right :) (I have never seen a GSL-SE front caliper in person) Further, the FCs with 4-pots had the same master as the ones with single piston...

Just a heads up for those looking at a 929 Master cylinder, The brake booster from a 2001-2003 protege is usable with a 929 Master brake cylinder I have read in the past and can be had for much cheaper at junk yards! I will look for the link to this. But this is the mod I am thinking about doing on one of my cars.

Yes, the '04 base Miata booster is also an option. The Miata vacuum bung is on the outboard side, while the Protege bung is inboard. Otherwise, I think they're the same part. The FB booster is 8.60" in dia, while the Miata and Protege boosters are 8.75" in dia, so they should fit next to our clutch MC.

Of note is both the Miata and Protege MCs are dual diaphragm, while the FB is a single diaphragm. This means they will provide more effective boost and lower pedal effort, which might be a good thing when using a larger dia MC.

My master cylinder didn't have the bore diameter cast into it, so I ordered a Centric 130.45104, which is 7/8". When it arrived, it turns out it was the same part that I already had installed.

I added 2 Lb Residual Pressure Valves to front and rear circuits, bled the system, and gave it a try.

The dead zone is greatly improved, but still not quite as good as my benchmark system (2010 Cadillac CTS Performance Model).

I ordered the 1" bore Nissan Patrol MC, and I'll try that next.

The pads play into this too. I'm using Centric ceramics right now. I like the feel of semi-metallics better, so I may change those, too.