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Mushy brake pedal - Printable Version

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Mushy brake pedal - pure&simple - 06-21-2011

I'm trying to form some theories as to the cause of my mushy brake pedal. From many conversations I've had, this seems to be a common problem yet to be solved.



What I mean by mushy brake pedal is this:

  1. First inch of travel provides no resistance and does not engage the brakes. Is there an adjustment to control this?

  2. Once the brakes are engaged, the feel is spongy. The harder I push, the lower the brake pedal goes.

  3. In a track situation where you are trying to heel/toe downshift, it is difficult to maintain even brake pedal pressure when blipping the throttle due to the brake pedal not providing a solid foundation under the ball of the foot.

  4. Even though I bleed the brakes before every event, brake pedal sinks lower during the course of a track event. A 3-day event is the most I've ever been able to do without re-bleeding.

  5. After re-bleeding, the pedal does come back up a bit, and is slightly firmer, but still spongy.


What I'm looking for is a firm pedal that is modulated by pressure, not travel. I would like it to move maybe an inch total from breathing on the brakes to ABS lockup. I would also like to be able to run several events between brake bleeds.



I have recently (last 2 years) had the following done:

I thought the possible remaining causes would be:

  1. Air bubble in the ABS controller. Unfortunately, there is no way to trigger the ABS while bleeding the brakes on the 968 like there is on some later Porsche models. You need to bleed, drive it and kick the ABS, then bleed again. Haven't tried this yet.

  2. Caliper flex. Our calipers are old, early 1990s technology. There must be stiffer options available.

  3. Non-floating rotors can't expand properly when hot (and the non-M030 brakes do get plenty hot on the track).


I just added another possible cause that I hadn't considered before - the brake bias valve. It is installed in the rear brake circuit to reduce rear brake line pressure relative to the front to keep the rears from locking before the fronts. It appears to do this by absorbing a percentage of rear brake energy in a spring. So we have a spring inline in the rear braking circuit - could this be the problem?

Mushy brake pedal - 94SilverCab - 06-21-2011

I'm no expert by any means...do you have S/S Brake Lines? When was your Master/Slave replaced (Bulging Clutch Line?)?



Jay

Mushy brake pedal - pure&simple - 06-21-2011

Good questions... yes to both. Edited above.

Mushy brake pedal - flash - 06-21-2011

what fluid are you running? you may need a higher temp fluid, or even a silicone one (though that means changing all seals)



have the calipers been done? you could have a seal shrinking with heat (yes, rubber acts backwards of other materials) and allowing air to pass by

Mushy brake pedal - pure&simple - 06-21-2011

I've tried the following brake fluids with similar results: Superblue, Motul 600, Motul 660. Edited above.



All 4 calipers rebuilt 2 years ago by RS Barn. Interesting theory about the seal shrinking with heat, but air is not part of the braking system. So unless the caliper is leaking fluid, the pedal feel should be unaffected, right? What am I missing?

Mushy brake pedal - flash - 06-21-2011

not exactly - fluid is much easier to hold back than air, especially hot air - when a seal shrinks, while it may hold fluid, it may not hold back air - heated air (like around a caliper) can slip past a seal pretty easily



likely the caliper seal area was either not cleaned well enough, or it was pitted and the seal is shrinking with heat just enough to let the air get into the system, thereby requiring repeated bleeding every time it does that



the master could do this too, as could any other moving seal



unfortunately the only way to find out is to take it all apart again

Mushy brake pedal - pure&simple - 06-21-2011

Ok, your theory is that air might be *entering* the system at the pistons through leaky piston seals. Have you seen this before on 944/968s?

Mushy brake pedal - flash - 06-21-2011

i've definitely seen it before on a LOT of cars with rebuilt calipers and/or masters - the 968 is nothing special or significantly different



rebuilding calipers is not as simple as it might seem, and the inspection for pitting and surface condition is key to it working - it doesn't take much to have a leak, and it isn't always easy to see when you are rebuilding them - i've always had to use a magnifying glass and a very sharp metal pick to go over the surfaces to look for anomalies



this is where having started out in british cars, with their horrible hydraulics, was a real advantage and taught me a lot about seals and such

Mushy brake pedal - pure&simple - 06-21-2011

Makes sense. Any thoughts on the bias valve as a probable cause?

Mushy brake pedal - flash - 06-21-2011

not likely, unless you used thread tape or something like that, which is a no-no - it's worth checking to make sure everything is tight though

Mushy brake pedal - JWahlsten - 06-21-2011

Are you getting fluid coming out of the master cylinder reservoir?



I agree with Flash and I have had multiple master cylinder failures in 951s/968s (same part). I know you replaced it, but it could be a bad part.



Did you replace the vacuum booster? Any leakage there?



I doubt it's the calipers, but it could be.

Mushy brake pedal - pure&simple - 06-22-2011

I've never had to add brake fluid - level stays constant. After a track session, I sometimes see moisture on top of the reservoir around the cap. Replacing the MC made no discernable difference.



Vacuum booster is most likely original. What sort of leakage might occur there?

Mushy brake pedal - flash - 06-22-2011

the moisture on the top is your clue - you are condensing somewhere because air is getting in



the vacuum booster should have no effect on the integrity of the hydraulic system - it will have an effect on pedal pressure, and could vary depending on its ability to hold vacuum over the range of temperatures



a thought occurred to me - it could be your clutch hydraulics at fault - they share the same reservoir, and if something is getting in there..............................

Mushy brake pedal - pure&simple - 06-23-2011

Thanks, flash. Will check it out.

Mushy brake pedal - AJG - 06-23-2011

There are special o-rings of superior quality material available for high pressure applications and may be suitable for cars. will update when I investigate a bit more.



Andy

Mushy brake pedal - AJG - 06-25-2011

[quote name='AJG' timestamp='1308866733' post='111644']

There are special o-rings of superior quality material available for high pressure applications and may be suitable for cars. will update when I investigate a bit more.



Andy

[/quote]



Viton V75 O rings are top grade parts, they are slighty stiffer if I remember correctly (all depending on Shore hardness) and may be useful on brake pistons.



Andy

Mushy brake pedal - jfrahm - 06-26-2011

Air is good at hiding in this system. The ABS pump and both halves of the calipers are good places for this.



An old motorcycle trick works pretty well on these cars, which is to compress the brake system overnight to make the bubbles very small and encourage them to migrate. On a bike this is super easy as you just wrap a bungee around your brake lever but in the car it's not so simple. It also may require disabling the brake lights.



What I have done is to place a block of wood across the lower front of the seat and use it, along with another 2x4 or other prop rod, to hold the brake pedal down. Adjust the seat to the length of scrap.



With the car in a position to bleed the brakes, compress the pedal and leave for some hours. Then bleed (one shot only) close, uncompress, recompress, wait, bleed, close, lather rinse repeat. It may take days to do all four corners depending on how quickly you go through the process. I do not know if waiting only an hour or two between bleeds is any better than waiting overnight but I find it easy to do this procedure every night and morning. It takes a while due to the multiple bleed locations on the 4 pot calipers. If you skip bleeding both, inner before outer, you will never get the air out of the inner caliper and you've got mush.



On my Audi I never got the damn ABS pump to bleed until I ran it using the computer interface during a bleed.



You can also get some good results with a pressure bleeder if you think about getting nice strong shots of fluid through the system that hopefully carry air with them. Gentle and slow flow just moves past the bubbles. That may be why the long procedure works well. The pressure is very high, the bubbles are very small, and the shots of bled fluid are very forceful.



Cheers,

-Joel.

Mushy brake pedal - pure&simple - 06-26-2011

Good points! Thanks, Joel.

Mushy brake pedal - AJG - 06-27-2011

Another point is that as the master cylinder is probably the highest point in the hydraulic system, when you bleed at the brake end it's very diffiucult for all the bubbles to migrate to the highest point or be released at the brake end. If reverse bleeding could be achieved then better results might be the outcome.



Andy

Mushy brake pedal - AJG - 06-27-2011

Just had a thought, if a pipe of about three feet could be hooked up at the brake end and held up in the air filled with brake fluid (kept away from paintwork of course) it might encourage bubbles to migrate to this high point.



Andy