Cloud9...68

As stated in my thread about my (formerly) sticky brakes, I've been fighting a mushy pedal for some time on my 968 with 944 Turbo S calipers. I can't say definitively when I first noticed it. The car is primarily a track car, and when an instructor drove it recently, he noticed that the pedal felt mushy. I hadn't done any work on the brakes in a very long time, other than periodic bleeding with a Motive pressure bleeder, which I've used very successfully on all my other cars. The pedal depresses several inches, until it is about even with the gas pedal, before the brakes start to grab. The stopping performance of the car is fine as far as I can tell, but the long initial pedal travel makes heel-and-toe downshifting less than optimal.

As I said, the car has 4-piston Turbo S calipers all around, with adapter plates to allow them to bolt to my stock (non-M030) spindles. It has Zimmerman cross-drilled rotors, and Porterfield 4S pads, with about 3/4 of their lining left, and stainless steel braided lines. I've recently replaced the master cylinder (no difference - didn't make the pedal feel any better worse). I just got done rebuilding all the calipers. The pistons and bores were incredibly clean - hardly a trace of any crud of any kind- they look like they came off a one-year-old car. The new seals I installed resolved the sticking problem - the brakes release fine now - but the rebuild had absolutely zero impact on my pedal feel. The inside of the fluid reservoir is also very clean.

Here's the bleeding technique I used, which was recommended by 67King: I filled the Motive reservoir with cheap, generic DOT3 fluid, and pressurized it to about 13 psi. I then opened both bleed screws on the right rear caliper until I got a steady stream of fluid coming out. I then got in the car, and pumped the pedal as hard as I could twenty times. I checked the flow from the caliper, and saw there was no air whatsoever coming out. I closed the RR caliper's bleed screws, and repeated the process on the LR. I then cracked the master cylinder's bleed screw, and got a few air bubbles, which quickly disappeared. I then did the same procedure on the RF, then the LF. I made sure there was at least 13 psi on the motive throughout the whole procedure. Then, reasoning there may still be air trapped in the rear, owing to the longer lines, I repeated the procedure on the RR and LR. So, I pushed A TON of fluid through the system, under very high pressure. The result? No improvement whatsoever. No better, no worse. Then in what was probably a mistake, I displaced the cheap fluid with high temp fluid from Millers Oils. Again, no difference in pedal feel. I wish I had kept the cheap stuff in there to continue troubleshooting.

I suspect I may have some air trapped in the bowels of the ABS system, but I have no idea how it may have gotten there. There are no visible leaks anywhere in my system. Are there any special techniques I need to use to get air out of the ABS system, beyond the aggressive bleeding technique I used? I doubt the problem is with my brake booster, because the brakes have about the same degree of mushiness with the car sitting with the engine off, and with the engine running. And yes, I installed the calipers with the bleed screws on top (I ran across a thread on a BMW forum where someone had installed their calipers upside down!). Overall, I feel like I'm completely missing the boat - there's something wrong that I haven't addressed in the slightest, despite all the work I've done. Any ideas would be appreciated.

jerome951

2 questions:
Can you clarify 'mushiness' vs. 'long travel'? I.e. does the pedal stay mushy through it's entire stroke, or just excessive travel for a bit but then the pedal gets firm?

Did the mushiness start after the calipers were rebuilt? If so, the new seals could be retracting some of the pistons away from rotors, leading to excessive travel. Not difficult to resolve and should go away once the seals get broken in.

If you've pumped that much fluid through, I'm highly skeptical there is still air in the system.

Cloud9...68

The pedal moves about 2" before the brakes engage, but the pedal isn't firm at that point - I can continue pressing the pedal at least another inch beyond the engagement point without too much effort. So the travel is long, AND the pedal is mushy. This happens with the car motionless with the engine off, with the engine idling but the car not moving, driving around town, and under heavy brake application at the track. The best way I can describe the pedal feel is that it feels like a brake pedal that belongs in a Buick. The brakes work, but they're anything but confidence-inspiring.

And the mushiness definitely didn't start after the calipers were rebuilt. The rebuild, while it corrected the problem with the brakes failing to release properly, had zero impact whatsoever on the pedal firmness/travel. Same exact result with the master cylinder replacement. It just feels like I'm totally missing the boat - like there's something else that's causing my problem that I haven't identified yet. Also, I can't identify when the problem started. I suspect it's been very gradual over time, and was brought to my attention by an instructor who drove the car a few months ago. So, now that I'm aware of it, it really bugs me.

Droops83

Did you buy the car with the bigger brakes installed, or did you install them yourself?

The reason I ask is: are you sure that the calipers are installed in the proper orientation? There are arrows cast into the caliper bodies (right above the PORSCHE lettering) that denote the direction of rotor travel. In other words, these arrows should be pointed upwards when installed on a 944/968 (both the front and rear calipers are installed at the rear of the hub). The upper caliper pistons are larger than the lower ones to prevent tapered pad wear; if the calipers are somehow mounted upside down (arrows pointing down, smaller pistons on top), the pad pressure will not be applied evenly to the spinning rotor, and braking force and pedal feel will not be ideal.

More info here:

http://www.lindseyracing.com/LR/Parts/BIGREDS.html

Check this front AND rear and report back.

E-man930

Are you tracking fluid level? (For fluid loss?) My buddy had a 965 with a mushy pedal and he had a leak at his ABS pump/distribution block that was preventing proper hydraulic pressure from building. You could have air in the ABS curcuit. ***edit - posting while 1/2 asleep*** You may want to pull your passenger fender liner and check for leaks around the ABS block/pump.

NM'87 951

when we replaced the brake MC we had an issue with the vacuum hose going to the bell housing, which earned her a set of replacement lines and clamps. Pedal firmed up.

Just a different point of view.

Cloud9...68

Droops,

The Turbo S calipers were installed by the previous owner, actually by a competent mechanic. It's time to rotate my tires anyway (LOVE my square 17 x 9 setup), so I'll take a look. I'm 99% sure the arrows are pointing up, though, but I'll confirm this weekend and report back.

E-man,

Yes, and I'm not experiencing any fluid loss whatsoever. But I agree that I may have air trapped in the ABS plumbing, but so far I haven't heard of any way to get it out.

NM'87,

Not sure what you mean by the vacuum hose going to the bell housing.

E-man930

Bleeding air out of the ABS housing / pump sucks. Have you thought about scrapping the ABS and converting back to the '86 hard lines? Only thing I can tell you is you will have to pull your fender liner, and then come up with a method to jumper or activate the solenoids to get the trapped air out while bleeding each channel at the same time (or else the air won't make it out). I think you will have to rig up some sort of way to get fluid to run through the system and maybe recycle it to the master cylinder reservoir until you know the air is all out, then bleed per normal.

Cloud9...68

Yes, I've seen some links online showing people hooking up some sort of contraption that activates the ABS solenoids on GM cars to allow bleeding of the system while it's tricked into thinking the ABS is on. I'm pretty much at the point of taking it to somebody who knows what they're doing. But I still don't understand how air ever got into my system in the first place, since I've never cracked open a brake line, except during the recent caliper rebuild, and the mushy pedal predates the caliper rebuild, probably by years.

Droops83

Well, one way to bleed the ABS is to go to a place where you can safely do a number of panic stops and cycle the ABS system repeatedly. The hope is this may result in any trapped air working its way out of the ABS lines and into the main brake lines. Then you can attempt to bleed the system normally and see if any air comes out. This has worked for me before on a few customer cars with early ABS systems that cannot be activated via scan tool.

It is certainly worth a try before attempting a more invasive procedure.

Cloud9...68

Since I track the car on a weekly basis, the ABS is engaged quite frequently. I tried bleeding the system using a different technique (pressurizing the MS with my Motive, and having an assistant pump the pedal and hold, while I loosened one bleed screw at a time). I got some bubbles this time, and the pedal feels a bit firmer. I'm going to repeat this a few times, and hope it gets the rest of the air out. But I'm finally making some progress. Still don't understand how air got into the system in the first place, though.

Oh, and I did confirm the calipers are mounted correctly - on the rear of each spindle, with the arrow pointing up.

Droops83

Cool, sounds like you're on the right track.

Sometimes it is very difficult to get trapped air out after component replacement. The manual/pressure bleeding method that you did is just the ticket, just keep doing it (with regular brake fluid, not pricey high-temp stuff!) and you might get it all out.

Be sure to fill the pressure bleeder partially with fluid, and ensure the car's brake fluid reservoir is full before you attempt it again. Also, tap each brake caliper with a soft hammer or similar while bleeding; sometimes this helps to dislodge stubborn air bubbles.

divil

Hey, I posted in your other thread that I had a similar issue, and that I planned to (1) try a reverse bleeder and (2) get a professional to take a look. So here is my update (sorry for the length):

The reverse bleeder did not give much improvement, if any. At first, the pedal was actually a little worse, but it came back and ended up feeling about the same on the street. At the track, it did feel a little better, but not enough to convince me that the reverse bleed did anything. I noticed that driving fast somehow improved the feel of the pedal - don't ask me how! In fairness though, I didn't put that much fluid through with the reverse bleeder (maybe a 1/4 of a bottle), and I will definitely try it again next time I need to bleed. I do think it would help in some situations. It's a very compact little tool that I will definitely be bringing to the track in future.

I then tried tightening up the front wheel bearings. They were adjusted in accordance with the factory manual, but there was still an awful lot of movement in them with this adjustment - more than enough to move the pads (you could tell by rocking the disc when bleeding - the fluid was visibly agitated). How the pedal feel can ever be consistent with the bearings adjusted like this, I don't know, but that is the correct adjustment according to the manual. In any case, tightening them up didn't help in the slightest.

I dropped it off at a shop that knows what they're doing, and they bleed it a few times, and eventually got a lot of air out. I just picked it up, and there is a huge improvement. It's still not perfectly consistent, and even when it's at it's absolute best, it's still not what I'd consider "rock solid", but it's definitely far better than I've ever had it before. For me, it's at the point where if I can sort out the little inconsistency that's still there, I could accept that maybe this is just as good as the pedal on a 951 can be. A little disappointing, but "livable-with".

One thing the mechanic mentioned speculatively when I dropped it off was that it might possibly be a failling ABS unit. He said they can stick open slightly and let some fluid back to the MC when they shouldn't. So that might be worth taking a look at, although I'm not sure how you could diagnose that.