Question: brake pedal height after single pump
 

The height of my brake pedal at first engagement changes if I pump the pedal once, and I was wondering if this was common behavior, or if something needs fixing. Without an initial pump, the brakes engage with the pedal a little lower (too low for heel-and-toeing). The brakes do work well, I don't get any fade and the pedal doesn't continue to sink, but it does feel less firm.

However, if I pump once first, the brakes engage significantly higher (just right for heel and toe), and do seem to feel firmer. I track the car, and have the wide-foot pedal extension, and will need to adjust the pedal height unless I can sort this out. Thought I would ask this question before I went to work.

I have a stock brake system. I've had bad master cylinders before so know what a sinking pedal feels like (and this isn't it). I've bled brakes lots of times and am pretty confident I am doing that right. I searched for similar questions, but found only this guy's post from long ago, and his question went unanswered: https://www.rx7club.com/3rd-gen-archives-73/newbie-brake-questions-14063/

Any suggestions as to the likely cause would be greatly appreciated.

BONUS BRAKE TIP: The best tool I've found so far for retracting the front calipers is this simple and cheap pony clamp - the Pony 3203HT shown at the bottom of this page.

http://www.internationaltool.com/jorgenso.htm

Take off one of the vinyl jaw covers and jaw is then thin enough to fit in, and it is possible to gently rock stubborn pistons enough to get them to retract without having to loosen the bleeder screw. You will need only two if you replace the pads one at a time. Looks a little goofy, but it works a lot better than a screwdriver (or a purpose-built tool I have, for that matter).

My FD is the same way. Lots of people over the years Ive been on the forum have reported the same thing. I know quite a few people have a habit of taping the brakes once before actually braking while at the track. I forgot what the term is called but some people recommend doing this to get better brake response.

There are two main reasons for the pedal to pump up. One is air in the lines, It can be fustrating to get all the air from the system, the power bleeders work wonders for this. The other is rotor run out, it doesn't take much to push the pads farther away from the rotor over a little time. You'll need to put a dial indicator on the rotors to check for this. I have adjusted my pedal all the way up to be able to heal and toe.

Thanks for the replies (you too, Wan!). Gadd, So I gather your pedal does NOT feel different on the second pump? I have speed bleeders coming, but have been getting by doing the bleeding the old fashioned way (with my wife or son's help). Slowly depress pedal, open bleeder on the downstroke, and close it before reaching the bottom. I've been assuming this is sufficient to remove air from anywhere in the entire system, including the master cylinder. Is this true?

http://www.stoptech.com/tech_info/wp...edbrakes.shtml

http://www.zeckhausen.com/bleeding_brakes.htm

easier to post a couple of links than it is to list the proceedure.

The way you have discribed works it just takes time to get all the air. This is an example of a power bleeder http://www.apexperformance.net/cartg...p?pid=49&cid=8. It's a good invesment if you track your car and are flushing the fluid often. Or you could make your own version like I did. My pedal does pump up but it's fairly minimal, about 1/4" .

All FD do this. The theories I've seen as to why the brakes act this way are wrong.

Theory #1: Brake rotor run out

It's not brake rotor run/ pad knock back because you can make the brakes do this while the car is parked motionless in the driveway. It's also not brake rotor runout because every single FD does it and not every one of them has warped rotors. If they all DID have warped rotors the brake pedal would pulsate madly anytime you used it.

Conclusion: It isn't brake rotor run out or pad knock back.

Theory #2: Improper brake bleeding

If it were improper bleeding or some other servicing issue the cars wouldn't have done this brand new sitting on the showroom floor (they did). If it were improper bleeding then the guys who DO properly bleed brakes wouldn't have this problem (they do).

Conclusion: It isn't air in the brake lines or improper bleeding.

My guess is the power brake booster. Cars with removed ABS, cars with aftermarket proportioning valves that replace original, cars with aftermarket calipers (big brake kits) and cars with 929 master cylinders still do it. The power brake booster is the only thing left unless the FD and 929 masters have some quirk that makes them act this way.

Mine does it as well, it really annoys me.

Wow that's a definitive statement.

I tried to get the pedal to pump up while the car sat idling the afternoon but I couldn't get it to do it, but I might not have given it enough time. I'll pay attention to that for a few days. The run out does not have to be much, with four pot calipers, moving the piston back just a small amount equals a relitively large amount of fluid. and I contest not enough to feel in the pedal.


Would you agree the air in the system would cause the pedal to pump up? And could be the problem primerGrey is having?


I don't see how the booster could cause this problem, in fact mechanically it would reduce pump up as I see it.

primerGrey you can give the brakes a quick stroke with your left foot before the braking zone. I know I'm not near coordinated enough to pull that off.

It's also true ;)

Runout can knock back the pads, but in order for runout to knockback the pads the wheels must be turning. If you can get a higher pedal after one pump in a parked car it isn't runout or knock back because the wheels haven't turned. I'll have to repeat myself and point out again that it would be folly to think that every single FD on the road has brake rotor runout and it can never be fixed no matter what's replaced and even though there's all this runout the brake pedal doesn't vibrate. It's not runout. You can take that to the bank.

So you contest that large amounts of fluid are being displaced but you can't feel it in the brake pedal? Okey dokey.

Air in the system can cause this problem, but it's not the problem these cars tend to have. I'll again repeat myself. If it were air introduced into the system these car wouldn't have acted this way when they sat brand new on the showroom floor (they did) and it could be fixed by properly bleeding the system (which doesn't fix it).

I have no evidence it's the booster other than process of elimination. People who have done brake mods have replaced everything else and still have the same issue. The booster works by applying vacuum to the front of the bladder which is in front of the brake pedal. This "sucks" on the pedal so you don't have to push it with your foot as hard. The booster needs vac only so there is a check valve to prevent positive manifold pressure from entering the booster. If the bladder in the booster relaxes away from the master then it will take an increased pedal stroke before fluid actually begins displacing in the system and building brake pedal pressure.

Ok, sounds like I should adjust the pedal for the FIRST pedal engage height, and not worry about the pump up. Thanks guys.

Is there anyone on this forum with a non-stock booster? It would be interesting know if that solved the problem.

If, by design, the system tended to slowly withdraw fluid from the pistons a bit after an actuation, wouldn't it have this effect? Just a random hypothesis - I haven't looked at the brake system design at all. I could imagine wanting to design such a thing in though, since it would reduce rolling friction.

Hmm, I don't think I am either, and trying to sort it all out could put me over the curve instead of around it ;)

You have no reason to repeat yourself, I read you the first time. All brake rotors have run out even brand new ones, this is not a perfect world we live in. Mazda says four thousands max, thats alot I don't believe I've ever measured one that far out. But with a relatively large volume calipers it takes a small amount of movement in the pads to equate to a larger amont of movement in the pedal. Mechanical advantage and all. There are many things in the system that dampen the vibrations from the brakes, hose, linkages the firewall itself flexes when you apply the brakes. [/QUOTE]


I don't see any reason for sarcasim. I think you are taking that statement out of context.

I never got a chance you sit in one of these car on the showroom floor. Air in the brakes may be primerGrey's problem. There is no way for either of us to know.


I think you have a misunderstanding of how a brake booster works. When the pedal is full up, there is a valve that cuts atmospheric pressure form the back of the diaphragm, so when the car is sitting the booster is doing nothing. And all boosters have a check valve thats how they store vacuum. Also the rod going to the mastercylinder can not pull on the piston it's not connected, it's a rod that sits in a cup. I don't see how your theory holds water.

Nothing changes.
I have this problem - changed the MC and it got a bit better.
But the brakes are not what they used to be.

So nine years later - do we have an answer?

Have you rebuilt the front calipers? Just an idea...

Not In A While
 

The brakes actually work fine - will activate the ABS.

Just not the way they used to be.
They used to be one small tap and up high and hard. Like most FD's

Sounds like air in the lines.

I have the same issue, I don't know if there is an FD specific issue with brake feel, but from my years of experience driving and wrenching on other makes, it seems like a combination of a worn out booster and an internal leak in the master cylinder. most of the dual diaphragm boosters I've felt have some degree of play that makes the pedal feel imprecise. The booster is a difficult issue to address since you won't be finding a brand new unit anymore and setting the yoke/ rod play (if the fd is even adjustable) is a tedious PITA. i opted for a MC rebuild kit and I'll have a look at booster adjustment while i'm in there.

Install a Mazda 929 brake master cyl and you won't have it anymore. Other than making a small line or having a brake shop make one for you it's a direct fit. It also makes the pedal firm. I have a new in box one from Malloy Mazda if you want. PM me and we can talk.

Mike

This can be caused by pad knockback. It happens when your wheel bearings start to wear out. You can verify this by braking, then, without putting any steering input in, accelerating and braking again. Is your pedal firm? Now try hard turns to the left and right. Is your pedal mushy now? If so, check your wheel bearings and replace as needed.

I don't think it's the MC.
It's passed every suggested test.
So ArmitageGVR4 I will try your suggestion.

So after rebuilding the master cylinder, flushing and bleeding the system, checking the booster push rod play, replacing pads and rotors, bedding and driving for a few days, I've come to the conclusion that the issue (at least mine) is not with the hydraulics. after trying a few different things with my driving style I've noticed a very firm and consistent pedal feel when I have rapidly transitioned from full acceleration (at peak boost) to full braking, or when I left foot brake. This all points to the booster. From what i can tell the booster is just slow to react to changes in manifold pressure, so when you apply the brakes after having already lifted off the throttle, the booster is assisting, and you get a lower pedal that requires less effort, with one or two pumps, the pressure differential from one side of the booster's diaphragm to the other is all but eliminated and the pedal firms up. then after a second or two of idle vacuum the booster assist returns. It's just quick enough at losing vacuum and just slow enough at returning to vacuum that it feels inconsistent.

I tried the pad knock back test.
It's not that.

thad try normal speed and brake with your left foot at the same time.
Let me know your results

I did this many years ago for the track and the difference is night and day.
Can't recommend it enough.

Just wanted to add my end of problem story and hopefully it will benefit other members.

My mechanic is Cam Pettit (ten minutes away, lucky me!)

Low brake pedal / MC passed Cam’s bad MC test / even though, we replaced MC with new MC / still had the same problem, maybe just a bit better / bought Zoom Zoom’s 929 (great price and thanks ZZ) / brakes are worse / Pettit felt the only other component was the ABS system / Pettit rebuilt the ABS unit / brakes are awesome, better then ever which he attributes to the 929.

The ABS was full of 23 years of crap to where the system could not function correctly.

I try to keep the mileage down and generally only drive it during cold fronts here in FL.
Not driving it keeps the mileage down but is not good for brake systems and probably other reasons.
And it turns your tires into stones.

Hope this helps.