JayG

My pedal is soft. It travels more than what I would call normal before you can feel the brakes start to grab

When the car is off (no boost) the pedal is rock hard
I have replaced the master cylinder and the system has been flushed and bled twice, so I an very confident that there is no air in the system. ABS works properly. The ABS pump has been tested with a PWIS and is fine

The brake lines are newer and SS. Pads and rotors are newer

When the car is running and you have brake boost the pedal is soft.

Our theory is there is too much boost.
Could that be the case?
Can a booster fail in such a way that it increases boost?

I need to get the wrapped up fast as I am planning on going to the track this week

I guess I could always disconnect the booster and go with manual brakes

Your suggestions are very welcomed

bekks

I once installed calipers upside down (on a Honda). They worked, but not well, due to not being able to fully bleed them. Took a while and a couple mechanics to realize the problem.

Macster

Air in the lines is often the cause of the symptom you describe.

Are you sure you followed the proper procedure for brake bleeding? There is an order which calls for the furthest distant caliper to be bled first. I do not recall which bleed screw -- inner or outer -- is bled first though. The screw is opened and the fluid needs to run out until it runs clear. Be sure you keep the brake fluid reservoir full to avoid getting any air in the lines or you'll have to bleed the system again.

Then one moves to the closest wheel and so on until all are done. Also, if the car is equipped with a manual transmission be sure to bleed the clutch slave cylinder, too.

If you suspect too much boost this would require I guess too much vacuum. Thus check the amount of vacuum at the brake booster at hot engine idle. I have no info on what the allowable range of vacuum is.

Last but not least I vaguely recall the booster can develop a problem that I seem to recall involves water getting into the booster. The culprit is clogged body water drains. In some cases the vacuum line can be removed and a check made for water in inside the booster. In other cases the booster has to be removed and checked for water.

JayG

Thanks Macster

The lines have been bled 3 times, I am fairly certain there is no air in them as we pushed 3 liters through. Clutch bled as well.

Good idea on measuring the vacuum, although I believe the booster regulates the amount of boost. There are different boosters for the 986, 987, GT#, etc all with different boost amounts.

No water problem.

Since with the booster disconnected , the pedal is rock hard, it's not the MC.

Don't think its the ABS pump/module as from what I know, the pedal would get harder, not softer if there was a failure or leak

I'm beginning to think it may be the pads. Gonna swap them out in the morning with some factory pads and see what happens.

Macster

Not sure how the pads can cause this symptom.

That the brake pedal is hard with engine off (and all boost in the boost reservoir used up -- generally this can retain enough vacuum to provide boost for one maybe two applications of the brake pedal with the engine off -- is normal. Most of us don't get a chance to experience unboosted braking. (I recall when my VW Golf TDi lost its V-belt things were seemingly ok until I got took the freeway off ramp. I had to really step on the brake pedal to get the brakes to work. Then after the light changed and I went to make a left hand turn I thought the steering wheel was frozen. I had to really pull on the wheel to get it to turn.)

A soft pedal with no air in the lines has me thinking the master cylinder is at fault. What can happen is during a brake bleed operation the brake pedal is depressed much more than it otherwise would be. This puts the master cylinder piston into an area of the cylinder into which it probably has never been before.

If the brake system has not been flushed/bled on a regular basis the virgin portion of cylinder can be quite nasty with corrosion/debris. (Even if the system has been flushed/bled on a regular basis this area of the cylinder can still be nasty and should be avoided.)

In pumping the brakes the piston works in this area and the seals are compromised.

Whenever I bled the brakes via working the pedal I was careful to avoid pushing the pedal all the way down to avoid damaging the piston/cylinder. (The clutch is different in that the pedal goes all the way down every time the clutch is used.)

JayG

Well after flushing the system 3 times including a vacuum bleed the pedal was still soft.
It turns out my pads were glazed and hard as rocks. I swapped in a set of factory pads and as soon as I tried the brakes backing out of the garage, the pedal was rock hard !

Since I was going to the track, I swapped out the factory pads for a set of Hawk DTC-60's
First time I hit the brakes, I almost went through the windshield. All I can say is Boxster brakes are AWESOME!!!

Macster

Having a hard time understanding how glazed pads -- and pads are always hard in my experience, they have to withstand who knows how much pressure per square inch and often at elevated temperatures -- can result in a soft brake pedal.

Regardless, I am glad to read you got the problem sorted.

JayG

We were a little confused as well as how the pads could cause a soft pedal
Basically, the pads were cooked and so hard they had no grip.

The pedal kind of felt like it was brake fade.

All I know is with the DTC-60's its stops on a dime and give 5 cents change

Macster

It occurs to me an explanation is the soft pedal arose from the ineffectiveness of the brakes due the "cooked" pads because this required more brake pedal pressure -- to little effect -- and this resulted in leakage around the master cylinder piston.

With the new pads with more grip less braking pressure is needed but the leakage potential is still there.

A "test" of sorts would be to with the car stationary apply the brakes. If the pedal sinks...

JayG

I understand your hypothesis,
The MC is new and is not leaking. The pedal with new pads is rock hard, with the "old" pads it is not.
If there was a problem with the MC, it would manifest itself with the booster disconnected as well as with it connected and I stand on the brakes, the pedal would sink, which it does not

Plain and simple, it was the pads.

I think I still have the pads, so if you want, Ill ship them to you and you can experience it for yourself

Macster

Sorry, don't mean to beat a dead horse, but I'm just still having a tough time accepting it was the hard pads that made for a soft pedal. Regardless I do not want the old pads.

JayG

nota a problem, my friend

I felt the same way

Sure you don't want the old pads???

JRitt@essex

It doesn't necessarily have to be the pad material that made the pedal soft, but it could still be the pads. Two possible scenarios:

1. Pad taper. When a pad friction puck tapers, all of the pistons no longer sit completely flush with the pad backing plate. Imagine the pad backing plate angled slightly, but the piston completely vertical like this...piston on the left, pad on the right ---> / I
That uneven force distribution can give the pedal a spongy feel. Some drivers mistake this as fluid fade late in an endurance race, but it's actually pad taper.

2. Pad backing plate bent/warped. If the pads were thrashed hard on the track, which it sounds like they were, the backing plate could have been warped. We've seen that happen on occasion. That results in the same scenario as above, but flip-flopped. In that case you have a flush piston pressing on an angled pad, like this ---> I /

Regardless of which it is, the result is a mush pedal. As the pistons extend out of the bores further, the pad wears more, and taper gets worse, that mushy feeling is exacerbated. I'm wondering if that's not what you felt. Either way, I'm glad you found the root of the problem!

JayG