I have a 929 master cylinder modification I performed many years ago. The brake booster I am using is used and repainted since my original one was quite rusty and tired looking. I matched the push rod adjustment to match my original booster years ago, but the brakes never seemed as good as the original master despite rebuild calipers front and back, Castrol SRF brake fluid, multiple bleedings, brake pedal adjustment, and new Hawk pads and higher-end DBA rotors. I recently checked the brake booster, which holds vacuum overnight and the check valve in the hose holds vacuum for several hours. I am finishing installing a new engine and now need to fix the brakes. I have the H-type tool that shows the pushrod is adjusted in spec without vacuum but the manual states this should be done under vacuum requiring a special tool. From everything I have tried to read on the subject over many different car forums, the pushrod can move enough to make a difference under vacuum.

I have tried to obtain the Mazda SST (special service tool) mentioned in the manual to no avail. None for sale on the secondary market. My local Mazda dealer doesn't have it anymore or won't admit they have it. Does anyone know if Honda master cylinders have the same cylinder size that inserts into the brake booster? Ours on the Mazda master cylinders is around 40 mm. The Honda SST can be purchased for around $100, which is not too expensive.

Any help would be appreciated.

Mike

P.S. I replaced the outer o ring on the master cylinder. If anyone needs a replacement, buy some from the O-ring Store. The size is 2.65mm X 37.5mm (NBR) Buna-N 70 Duro Metric O-Ring.

 

What exactly do you mean by “better?” Why did you go with this bigger MC to begin with?

 

This 929 master cylinder is a common modification many of us made starting about 10-15 years ago. The stock FD master cylinder is a 7/8" diameter piston and the 929 is larger or a 1" piston. Therefore, the 929 moves more fluid (larger volume) and will obtain higher pressure with stock calipers and lines. If aftermarket calipers, this is even more important since these calipers require a larger volume of fluid to work well. The stock master does not achieve full potential stopping power with larger, often time, 6-piston aftermarket front calipers and 4-piston rear calipers due to their larger volume requirements. Again, with stock calipers, the pedal movement should be less and stopping power more with the 929 master cylinder.

Mike

 

I know, I was wondering why you in particular did the mod. Do you have big brakes? Did you want a stiffer pedal?
In what why was the stock master better?

The stopping power isn't going to be better with a bigger master cylinder, it will be worse. You're effectively made the brake pedal shorter.
Everything else held the same, a smaller MC will have better stopping power, but at the cost of a squishier brake pedal.

 

Twofold at the time. First, I was going to go with bigger brakes back then, but I have yet to pull the trigger. Second, I drive in an usual seating position. I am kind of short, around 5"9" but I like my seat very far back since my arms are somewhat disproportionally longer than my legs. I also like my elbows locked out on the steering wheel. This means I use more of the ball of my foot and toes on the gas and brake pedal, which means the less pedal movement the better. I also adjust the brake pedal as far up as it can go. It's a quirky personal thing.
Mike

 

FYI, I did order the brake booster/master cylinder SST for Honda's. From my research, it appears to have the same diameter to fit in our brake boosters. I will let everyone know if it works. If so, it would allow more precise adjustments on our brakes. Hopefully it works. Remember, the spec from the shop manual on this is quite tight, 0.1-0.4 mm between the pushrod and the MC core. From my research on other car forums from various OEM's, applying vacuum can change a pushrod measurement by as much as 1-2 mm. This would potentially leave a large discrepancy if vacuum was not applied to do the measurement, which cannot be done without the SST.
Mike

 

Ok, you wanted a rock-hard pedal. You still haven’t answered the question. In what way was the stock MC “better?”

 

I think there is a misunderstanding. My statement above about the 929 master cylinder never seemed as good as the original master cylinder is why I think there is a pushrod adjustment problem on the changeover. The 929 should have been better but wasn't. My brake pedal would go almost to the floor on very hard braking after the changeover. Antilock would not even kick in. That's why I am still trying to figure out why. I hope that clarifies my statement.

Mike

 

Larger MC piston means LESS fluid (line) pressure with the same pedal pressure. Line pressure is force/(MC piston area). That also means less braking torque with the same area caliper pistons.

It also "should" mean a firmer pedal, as you said. So, is the pedal travel just too much but with a solid feel, or is the feel mushy? Mushy almost always means air in the system or something (brake lines balooning, calipers flexing, pads worn crooked, calipers not aligned with the rotors, etc.) flexing too much in the system.

 

I would describe the pedal fell as pretty solid later in the travel with the initial pedal travel seemingly to much before becoming rather solid. I have the pedal pushrod adjustment at the floor as tight as possible before engaging the brake booster and the pedal is quite high above the floor also with the brake light switch adjusted to the almost to the max outward. That's why I believe the pushrod adjustment may be the problem. I did power bleed the system several times with the use of speed bleeders on all four calipers. The pads are new and again the calipers are rebuilt with no obvious leaks. The brake lines are aftermarket braided stainless steel. If the pushrod adjustment is not the problem, I will have to look at these other possibilities.
Mike

 

So you are saying there's effectively too much free play, despite adjusting the pedal rod below the proper 3-8 mm?

I suppose it could be the push rod adjustment. Being a millimeter out of spec would introduce several mm of extra free play, depending on the lever ratio of the pedal.

I don't think you really need the SST to see if you're way off. Use some calipers, depth gauges, or what have you.

The rod should essentially be touching the piston when the engine is on. If the math doesn't add up to basically 0, you're probably off by a lot.

I wonder if plugging the MC with 10 mm bolts and seeing whether the fluid level in the reservoir rises when you bolt it to the booster wouldn't tell you whether it's touching... probably not a precise enough measurement.

 

Yes, to the first question.
I have the H-type MC gauge you see all over the internet. It is set perfectly with this. The problem again is that it is not under vacuum since the MC seals the brake booster to allow vacuum. If indeed vacuum can charge a pushrod distance by 1-2 mm, that would potentially put the pushrod adjustment way off the spec of only 0.1-0.4 mm. We will see if the Honda SST works. I will report back.
Mike

 

Do you have a hand pump? It'll take a lot of pumps but you can get it up to 500mmHg/19.7inHg specified in the manual.
I just did a booster swap on another car and there wasn't a measurable difference with vacuum using the H shaped gauge tool and calipers.

First post says the 929 master cylinder was swapped years ago, has this pedal problem been ongoing since then or has the car sat a while?
If you keep pressure on the pedal while stopped does it keep sinking? That would point to the MC being bad.

 

you can't pull vacuum on the booster without using the SST or installing the master cylinder.

 

I forgot that's what the o-ring is for.

 

No. Sealed by the o-ring on the MC, hence the need for the SST to test under vacuum as the manual states. I don't think the engineers would specify under typical vacuum conditions unless it was important. There would be no need for the SST is that was true. Why go through the hassle of the SST if simple measurement calipers or the H-type gauge was sufficient?
I have an electric vacuum pump (used to evacuate Freon from HVAC systems) that quickly evacuates the chamber in seconds. The brake booster holds full vacuum overnight. Good brake booster.
The 929 MC was installed years ago but the problem was there from the start. Also, only about 2,000-3,000 miles on the MC. Pedal will not continue to sink. Good MC.
Again, no air in lines due to multiple bleeds with even a power bleeder and speed bleeders on the calipers, which prevents air back in the system, when brake pedal is released.
Mike

 

yeah, that's why I said.

potentially relevant recent thread: https://www.rx7club.com/3rd-generati...shrod-1165057/

 

Sorry. I took your response in the form of a question, not in the form of a statement. I wasn't trying to be confrontational.
Mike

 

For what's worth... I went through adjusting the brake pushrod on my non turbo 2nd gen Rx-7 many years ago. It was as described, basically a long travel (longer than I preferred) before the pedal firmed up (had fresh steel brake lines and new pads with the 2nd gen 4 piston calipers). I did it iteratively rather than try to set it to some service manual spec. The first time I did it, it was basically too long (if I remember correctly), and the brakes were sticking because there was zero travel. So I turned it back the opposite direction a bit and it was fine.

I basically fiddled with it until I was happy with the pedal feel and there was no dragging of the brakes.

 

I am also having issues with the 929 mc. I don’t think the stopping power is what it used to be and I also have not been able to engage the antilock.
sorry I did 626 not 929 mod

 

Hopefully, the Honda SST I purchased will work on our brake boosters. I should have it in about one week and I will report back as to what I find.
Mike

 

Update. The Honda SST for the brake booster came and fits great and the booster holds vacuum with the SST in place. I will do the assessment and if necessary, the adjustment of the pushrod in the next few days and will report back. I will do the assessment with and without vacuum to see if it makes any difference in the measurement. If it doesn't, then the SST would not be necessary, and the simpler H-type brake booster pushrod adjuster would be all that is needed.
Mike

 

There's going to be a difference, since you can apparently pull on the rod and it will move a few mm out. There's some amount of play in the system at rest.
I'm curious whether there's a difference between simply pulling on the rod, and applying vacuum.

 

Yes, you are right, there is some play in the system with the ability to pull or push in the rod slightly. The SST and the H-type tools are supposed to be adjusted until the rod is pushed slightly back to be snug but not actually engaged enough to move the whole mechanism back toward the brake pedal. From what I have read, the large diaphragm inside the brake booster will move with the application of vacuum. How much, if any, it moves the pushrod I don't know. Again, I would believe it would have some effect on the pushrod, hence the need for the SST rather than the simpler H-type adjustor. After testing the movement with and without vacuum, hopefully I will have the answer. It may be just the difference you mentioned in the play. I will report back.
Mike

 

I had some time before going to a 4th of July party, so I did the testing using the SST. The SST is spring loaded so as you spin the adjustment wheel, and the internal piston will keep tension on the tip of the pushrod. I started at no vacuum and adjusted the small brass wheel until no gap is present. The SST internal piston is therefore snug against the pushrod. After 19.5 mm HG using a vacuum pump, the gap widened to between 0.15 and 0.20 mm using my feeler gauges. Therefore, vacuum caused the pushrod to extend toward the MC by this amount. It's not much, but since the spec for the recommended gap is between 0.1-0.4 mm, it could put one out of spec since it is about 50% of the spec range. For my car, I found out that my pushrod is actually currently too long rather than short (which is what I thought given my perceived free play). Using the feeler gauges between the SST (once I removed it from the brake booster), it is long by about 1mm. I will have to shorten the pushrod by 0.6 to 0.9 mm to bring it back into spec.

IMHO, the takeaway in practical terms, is anyone changing their MC or upgrading their MC to a 929 or something else, could use the H-type tool to adjust the pushrod at first and simply shorten the pushrod after adjusting to the H-type tool determination by approximately 0.25-0.30 mm (spec gap of 0.1 mm and the vacuum adjustment of 0.15-0.20 mm).
Mike