Viperbob1

Have a race car (it has been dedicated past 3 years) that all of a sudden the brake pedal when car is running (even just sitting) is super soft and goes to the floor. You have to pump it up to get any pressure on pedal and then easily still will go back to floor. When car is turned off, pedal is ROCK HARD. In the past even when car was turned on pedal was extremely firm. All good new fluid, Motul 660, bled several times after this started (and after every event). Also bleeds normal with good flow to all bleeders. Tried Pressure Bleeding and using the brake pedal. All calipers look good with no seals leaking. Lines are all in good shape. Changed master cylinder and no effect. Have not seen ABS unit go out on these like this, but would it cause this issue? No codes in the ABS unit and no ABS lights. Very odd. Any thoughts?

Veloce Raptor

When you bled system, did you also cycle the ABS valve to bleed it as well?

LC MotorSports

You can try cycling thru the ABS unit and see if you get any more air out of the system. Is this a Cup car or Street conversion into track slave (ask from the avitar). If its a street conversion... Did you bleed and flush the clutch? They share the same reservoir and can cause the system to get goofy... What it sounds like is a possible Vacuum leak in the Booster pump.

Matt Romanowski

I'd agree with the vacuum leak since it is ok when not running. Weird way for a pressure booster to fail, but it's possible. Can you unplug the ABS unit to see if it changes the pedal when the car is running? That would rule it out, then the only thing left is the vacuum booster.

Viperbob1

Did not cycle ABS unit. Been bleeding car for 3 years and never needed to (would have done it yesterday but did not have PC with me - DOHHHH). Also this CHANGE occured on track about half way through a Sprint Race. All of a sudden pedal to floor. Odd since using 660, good ventilation, and not hot day. Was last day, last session so we just put in trailer. Thought just must have boiled fluid somehow (though never happened before).

Was 997S street car. Ignore Avatar. Tranny was swapped just recently and clutch is bled after every event. Leaning towards leak in pump somewhere. If leak in vacuum leak in brake booster, then would not have power assist and pedal would be hard not soft. Apparently also this unit did fail a few years ago and was replaced. Very strange. Have not seen a lot of failing ABS units like this. No codes or anything.

LC MotorSports

I agree that typically the booster failure results in a hard pedal... But, I have seen it happen where when they go you end up at the floor with no pedal. I really do not believe it is the ABS unit...

2BWise

I agree with you. A vacuum issue would cause a hard pedal not a soft one. My first guess would have been the master cylinder, but since you've already replaced it it would be tough to assume the new one is already bad. What is obvious is that, with no external leaks, you are pushing fluid past a seal or valve. Personally, I have seen ABS valves wear (and or fail) to the point where the driver can push fluid past. It is difficult to diagnose if you don't have pressure taps installed. The symptoms were very similar to your description. Any brake apply with the vehicle on and the pedal slowly falls to the floor.

Veloce Raptor

In 3 years you have never cycled the ABS unit while bleeding? In many cars, it is a prime collector of brake fluid crud and needs regular cycling/flushing....especially given th eduty cycle that car is now seeing. IMHO....

Viperbob1

Dont cycle on every bleed. Usually beginning and mid season. Never seems to make a difference. No change is fluid coming out over normal. Maybe amount we bleed flushes system anyway.

Dan Jacobs

Change the ABS unit, get latest flash

LC MotorSports

Dan, this is a seperate question that may or may not be related to the OP situation. Would the latest flash cause trouble when they review your ECU data during a Scrut... or would they over look that for the stock class groups?

Land Jet

Would someone be so kind as to tell me how you cycle the ABS when flushing or bleeding? Thanks.

onefastviking

I presume Dan is refering to the latest brake ecu flash. The ABS ECU is separate from the Engine ECU, so if all PCA checks is engine ECU, which I am guessing is all they are doing then no one would know the difference. Ok, only a few people would know the difference. It also depends on "which" ABS unit is in the car.

onefastviking

Normally done with PWIIS (porsche computer) although can be done a few other ways as well.

onefastviking

It seems like you have checked all the likely sources and I would also lean towards a vacuum or ABS pump issue as well.
But just on a wild outside chance....... have you checked your wheel bearings for any play ?