imstimpy

I just installed 997.2 GT3 calipers and stainless brake lines. The entire system was pressure bled twice prior to driving; the second time was after noticing a leak at one of the newly installed lines. On the test drive and rotor/pad bedding the system operated properly. After parking I noticed I could pump the pedal to raise engagement, which has a small amount of compression then becomes firm, but the pedal slowly dropped to the floor.

The slowly dropping pedal didn't exist prior to the brake overhaul. Though I initially had a small leak, subsequent inspections cannot find any further leaks.

Any suggestions for where to go next?

Thanks!

Pep!RRRR

Is the cap tight on the master cylinder? Sounds like it’s loosing vacuum.

A432

Did you have the dealer or a shop with a PIWIS activate/bleed the secondary system? If not, you probably have air in the ABS.

pepr, the cap being tight doesn't matter. It's actually vented so fluid can flow. That's why track cars rubber band a rag over it so the venting doesn't lose fluid under high loading.

Also, vacuum always bleeds off when the car is off. The secondary system bleed is a common item since the valves have to be activated and durametric doesn't do it.

Dave in Chicago

If there is loss of hydraulic pressure, it's going somewhere. The obvious places to look are any that were disturbed during the recent install and bleeding process (new calipers and their fittings, new lines and their fittings, all bleed screws that were operated, reservoir cap, etc.). I usually hook up the Motive Power Bleeder and dial in about 20 psi, with plenty of fluid and no risk of letting the reservoir level drop to a point where air could be introduced into the system. There should be no pressure loss until I open a bleed screw, even then it is pretty slow and measured.

Then non-obvious option could be whatever wasn't touched during the process, that is the clutch slave. I always bleed that one last as part of the bleeding process. It's a remote possibility, but if fluid pressure is going somewhere... who knows.

As noted above, one could get one of the required devices needed to cycle the ABS unit for bleeding to ensure air is not trapped within same. I've typically not done so and cycle the ABS manually to ensure fluid is passing through it and it's chambers normally.

Good luck!

85Gold

Pedal going slowly to the floor without loss of fluid means somewhere in the system there is a bypass. The old standby was usually a MC when the internal cup seal was leaking. Nowadays with the ABS system providing brake proportioning as well as other functions I would lean towards air in the ABS system.

Peter

danschy

I have not done much to the brakes on my 07 Cayman, but I've worked on a lot of cars over the years and the symptom you describe is typical of fluid leaking past the seals in the master cylinder. You didn't mention anything about the master cylinder, but I have heard that some master cylinders do not take kindly to depressing the brake pedal all the way to the floor - apparently can mess up the seals. I've never experienced it - just a thought. Typically air in the system causes a low pedal, not a slowly dropping pedal.

MJBird993

I had a not-very old Jaguar that I bled the brakes (preventive maintenance) and somehow (I forgot what I did) got air in the system and could never get it all out. Couldn't figure it out - finally the guy that bought the car from me took it to the dealer and they replaced the MC. So I presume that all of that pumping did something that messed-up something else.

On a 997, which is starting to get some age on it, I can see where this could be a likely culprit.

Voyager6

I agree on air in the ABS unit.

However, you might want to take a mechanic's stethoscope and listen at the brake booster for a vacuum leak. Pedal will drop as vacuum in booster leaks out. Check at the large vacuum hose to booster connection and around the master cylinder to booster sealing area (O-ring at MC/Booster interface.) Easy enough to do.

V6

SpyderSenseOC

Pedal will actually get hard if booster vacuum leaks out.

Should never go to the floor without a leak/bypass or air trapped in the system. A pedal that is initially high and firm and slowly drops to the floor is usually indicative of a fluid bypass.

imstimpy

As most cars have a separate system for the clutch hydraulics I didn't even consider bleeding the clutch. I'll circle back to it later.

I checked and re-checked everything (again). There is positively no fluid leaking externally along the braking system.

I used a PIWIS to perform the brake bleed procedure. The process was rather involved and worked through another half liter of fluid (total consumed now is one liter). The pedal seemed to improve as I worked through the process. Since I was cycling the pedal I couldn't see if any air was bled. The color was wrong so I knew I was getting old fluid. Now the funky part: a very solid pedal when the car is off but turn it on and the pedal slowly drops. Being that it feels solid while off, my guess is the ABS/ABD is managing the pressure electronically when on. I recently recoded the system to be PCCB, which could be the cause I suppose.

After passing that much fluid through the system, I had the confidence to take the car to the track. On track the braking system worked wonderfully! I could feel ABS kick in when the chassis was upset or stop without ABS when taking a smooth approach. The pedal position was correct during heel-toe downshifts as well. Park it, though, and pump the pedal and it will hold firm briefly then slowly drop.

ramans

Is it worse than normal? On all of my modern cars they do this to some extend especially when not on. but if you keep your foot super hard on the pedal it will keep just going down, I've never had an issue with it on the street or the track, but both my 981s do this as delivered.

Dave in Chicago

One more thought would be compatibility of the currently installed parts. Is it possible that the GT3 front caliper configuration is ill-matched to, what I assume is still installed, a stock master cylinder?

On my 968, I upgraded the brakes to the M030 option (factory big brake) by installing the 928S4 front rotors and calipers, plus the proportioning valve appropriate for same. Both of those configurations use the same rear brakes and master cylinder, per the Porsche parts PET diagrams. And both configurations performed exactly as expected. In either configuration, they provided factory compatibility.

Okay, I'll stop scratching my head now...

mhm993

You need the bigger gt3 MC. Resolved this on my 987s.

xsbank

Master cylinder is on its way out. I had these symptoms on my 944 and it took a rebuilt mc to fix it.

imstimpy

I have the stock 981 S master cylinder in place still. In prior builds, increasing the fluid demand of the calipers without increasing the fluid displacement of the MC merely results in a softer, deeper pedal. The 997 GT3 calipers with the 981 S MC feels about like that: a softer, deeper pedal. I suspect the 991/GT3 MC will firm up the pedal but I'm skeptical it will have anything to do with the slowly dropping pedal.

The 9,000 mile MC? This behavior ONLY exists when the car is running. With the engine off the pedal is rock solid and will remain rock solid. After pumping it solid, then starting the car, only then will it slowly drop but only half way. If the engine is running and I pump the pedal solid, there will be a brief delay and then the pedal will slowly drop.