crazyfrog

Got a brake failure last year on the Nurburgring. Was lucky it was at Brunchen so plenty of room there.....Came down the hill with some serious speed , stept on the brake, the pedal was solid like a bric right from the top.....I had a couple of local germans in their GT3's right to my *** at that time !!!! AHHHHHHHHHHHH
Took that long bend without braking and keep opening the tap and I got away with it !!!! I pulled over straight after and slow down , did brake again and it was fine ?? I tried to understand what happend later on , and thought I did run out of vacuum due to a leak or something. Did few more laps and it was fine.
I since replace the one way valve pipe front of the servo . The fitting on the servo is tight in the rubber seal but on the other side on the aluminium plate it seems to be a little bit "loose" . It won't come off but it is a bit floppy. Is it suppose to be like that ? Is there any pressure test that can be done to check for vacuum leak on these car ?
If I have a fail again I could be unlucky this time ....
I had my ceramic at the time .

CarlosCA

Just happened today as well, '02 996 C4. No damage fortunately but a little unsettling. Appreciate anything we can learn from the forum here.

Macster

Doesn't read like a vacuum leak in the traditional sense.

Reads like a valve problem.

IIRC (and as brief as I can be) when the brake pedal is not being depressed both sides of the brake booster diaphragm are exposed to atmosphere pressure.

When the brake pedal is pushed this exposes one side of the brake booster diaphragm to engine vacuum. The other side is exposed to atmosphere pressure.

This then provides the brake boost we are all familiar with.

Upon releasing the brake the vacuum is closed off and atmosphere pressure is present at both sides of the brake boost diaphragm.

What it reads like is when you depressed the brake pedal the valve that puts engine vacuum to the booster side of the diaphragm or perhaps the valve that feeds atmosphere pressure was not closed off.

The result is no brake boost.

The result can be a solid as a brick brake pedal. The brakes still work but one has to stand on the pedal. Given how accustomed we are to this boost we are not prepared to do this.

(Lost power steering on my VW Golf one day when it tossed a belt. I can't recall now a brake issue but I do remember being caught by surprise when I went to make a sharp turn and found there was no power steering. I had driven maybe 15 miles on the freeway with no power steering and never had a hint. Lane changes went well, and the mild steering wheel adjustments to take a freeway off ramp were not a problem. But after coming to a stop at an off ramp red light then having to make a 90 degree left hand turn I did my usual attempt at turning the wheel and it felt like the wheel was locked. I then realized how much the power steering system contributed to the ease of steering the car. I grabbed two big handfuls of steering wheel and turned with considerable might and managed to get the car turned and around the corner ok.)

Anyhow, the valving that controls the flow of vacuum or atmosphere pressure is in the brake master cylinder/boost assembly.

My take is this is going bad perhaps the heat of the brake fluid having something to do with the symptom appearing.

If I read your post correctly the car is equipped with PCCBs. What can happen is since these do not fade the driver stays out far longer than he would with iron brakes and the brake fluid (among other things) gets very hot. It is this hot brake fluid that plays a role in the behavior you experienced.

It is important that the brake fluid be very fresh even with iron brakes but even more so with PCCBs for as I mentioned above brake fluid temps can be quite high.

Since the symptom was short lived, went away after you pulled over and slowed down and didn't reappear because you probably were tentative on the track after your scare (understandably so) this gave the brake fluid and hardware a chance to cool down some.

If there were a full time vacuum leak I think you'd know it from the engine's behavior either just idling or idling and you working the brake pedal.

But I'm not an expert. My advice is whenever a safety critical system acts up and especially if the acting up is short lived and apparently corrects on its own, my advice is to have the safety critical system checked out by an expert. In this case you would take the car to a qualified shop and report what happened and have the braking system checked out.