IceWater

I'm at my wit's end here. I've had a problem with my 968 (non-M030) where there has been about 3 inches of dead travel at the top of the brake pedal, like there's air in the line. I bled the system multiple times, but no dice. Replaced the master cylinder, installed stainless steel lines, turned the front rotors, replaced the rear rotors, changed the pads, bled the system 3 times (using a motive power bleeder), and I still get the sponginess. One interesting thing to note is that if I depress the pedal while its bleeding, it feels firm all the way (but that may just be because the system is under pressure the whole time)

Anyone have any ideas? Is it possible that air could be trapped in the ABS system or proportioning valve? Anywhere else? This is really driving me nuts...what's a Porsche with crappy brakes? And the first autocross is coming up soon!

Thanks,
Tom

pete944

Last night I installed speed bleeders and flushed the brake fluid with the Motive bleeder. I didn't have any problem at all. I never had to touch the pedal.
Could you have a bad master cylinder?

Rod in Orlando

You might want to check the vacuum booster for a leak or other malfunction if the spongy feeling occurs only when the car is running.

MHT

I have seen a similar problem in an early 944, ran gallons of fluid thru the system but it never got any better. The problem turned out to be the booster, I took the booster apart after replacing it and it looked fine, I could not see anything wrong with it, but the pedal was great as soon as the new one was installed.
in your 968 there is also the possibility that a valve in the ABS unit is stuck open allowing pedal pressure to act on the accumulator in the system instead of the calipers. You might try to exercise the ABS system, if you can find some slippery road surface, or maybe try tapping on the ABS assy. and see if you can free it up. When did the problem first appear? If it happened after a pad replacement during which you pushed the pads back without bleeding off the fluid, pushing the fluid back up the line, then I would vote for the ABS as being the cause. Good luck Mark

IceWater

Hmmm...

I've never worked on a brake booster before. I thought that they worked in parallel with the pushrod, not in series, so that there is always a direct connection between the pedal and the cylinder. If this is the case, then a problem in the booster couldn't allow any free play. Am I wrong about how they are set up? Wouldn't be the first time...

The ABS issue is worth looking into. I'll take her to a dirt road and fire up the ABS. I don't think I've ever pushed in the pads without the reservoir being vented, but who knows. Could you explain a little how a valve being stuck open could occur and how it would manifest?

Thanks a bunch guys,
Tom

adrial

How thin are your rotors?

I also recommend kicking in the ABS a few times...you can do it on a paved road...no need to go to a dirt road

Does anybody remember the test for the brake booster?

IceWater

Ooo...there's a test for the brake booster? That would be great!

The rear rotors are new. Not sure how thick the front ones are, but I just had them turned and the machinist measured them before doing so, so I'm pretty sure they're within spec.

Think I'll stick to a dirt road...may as well save the rubber. Plus, I just installed new metalmasters, and given how weak they are before being broken in, I don't know if I could get the ABS to kick in on pavement!

adrial

Icewater, you definetely want to break in the pads before you make any attemps to kick in the ABS. You could glaze em over if you don't...

I think its a series of nice gentle slow stops from 60 or 80mph down to 20 or so??

adrial

found the test over at Pelican! Thats where I found it initially...well thats where someone posted it initially.

with the engine stoped depress the pedel several time travel distance should not change

depress brake pedal and start engine pedal should move down a little

depress the pedal and shut off the engine hold the pedal for 30 sec there should be no movement

start the engine run for about 1 min then depress the brake pedal several times travel should decrease with each aplication

Good luck!

MHT

Tom, when I was talking about pushing the pads back, I meant that if you do not open a bleeder screw as you push the pistons back, you will push fluid from the caliper back up the brake lines toward the master cyl. Unfortunately this fluid can take with it some of the crud that builds up in the calipers, this junk then gets stuck in the ABS controller. Ideally one would open a bleeder screw and crimp off the brake hose with a pair of smooth jaw locking pliers. Mark

IceWater

Thanks Adrial and Mark.

I've heard many conflicting opinions on breaking in Metalmasters. I thought most agreed that several hard stops from 60 to 20 was the way to go, though.

I will definitely try the booster test tonight. Even though it would be an expensive fix, I'd be thrilled to have the problem nailed down.

I've never heard of openning a bleeder when pushing the pads in. Makes sense, though. I certainly haven't done it that way. After breaking in the pads I'll engage the ABS a few times. Is it possible that air is trapped in the controler (in which case, I should bleed again after engaging it, right?).

Thanks again, guys.
Tom

adrial

Icewater,

After a few autocrosses (kicking in the ABS a bunch of times) my brakes felt like they firmed up a bit...this is after I bled the brakes a few times and replaced the master cylinder.

Just see how it goes...if you search the archives you'll see that the air may just work itself out.

IceWater

I did experience what you're talking about once before...I had spongy brakes at the beginning of an autocross, but by the end they were firm. If there's anything to that, hopefully engaging the ABS a few times will have the same effect. I suppose if that works I should immediately bleed again to oust whatever air was purged from the ABS, huh?

Tom

IceWater

I found the booster test procedure a little unclear. This is how I interpreted it:

with the engine stopped, depress the pedel several times. travel distance should not change.

depress brake pedal and start engine. pedal should move down a little.

with engine running, depress the pedal, hold it there and shut off the engine. hold the pedal for 30 sec. there should be no movement.

release brake and start the engine. run for about 1 min then depress the brake pedal several times. travel should decrease with each aplication.

Is this right?

Thanks,
Tom

adrial

Your interpreation of the booster test sounds good to me.

I think all those steps are meant to be done in a series. This seems to be most important for steps 1 and 2. (each paragraph is a step)

I don't know about bleeding right after engaging the ABS...I never did and the brakes seemed to have gotten better regardless. If the air bubbles make their way back to the master cylinder...then the air escapes.