In a hot day in heavy traffic yesterday...
It finally started moving again and I was going about 40 MPH, when traffic slowed down again. Attempting to brake and the pedal fell to the floor, repeated pumping on the pedal and no change. I was able to avoid hitting anyone by changing lanes and using the e brake.
Then the brakes came back after a minute. Pulled over at an auto parts store to check and top off fluid, but it was full.
Tested the brakes today and they are fine, pourpusely dragged the brakes to overheat them for a few minutes on an empty highway untill they smoked, full pedal ... High speed brake, full pedal, full brake force. Lost the brakes in traffic later again.
I had a master cilynder go bad on me before and it came gradually with warning.

This is intermittend and I am not sure what to check.

I will do a full brake fluid flush and check all system. No apparent leaks though.

Any ideas?

 

How old is the brake fluid you have now?

 

Brake pedal linkage?

 

warped rotors? (but probably not, since you were not able to pump up the pedal)
sounds like mastercylinder could be really bad, especially, since one side alone didnt grab. (i.e. calipers are probalby fine). master might be toast

 

Ballooning brake lines?

 

I knocked loose my ds front brake line while changing the rack and lost all brakes. This is not your primarly problem, but it is suprising that, on the S4, if you loose one wheel, you loose the whole car.

 

I think its time for a new master cylinder , glad you didnt hit anything.

 

How can a master cyl fail intermittend?
It felt like air in the system...

 

It is strange.

 

Good question!!

 

you could always try a brake fluid flush if you dont get a good pedal after this then replace the MC

 

Brake fluid is hydroscopic, it absorbs water, so the question on how old is the brake fluid is a good question relating to the diagnostics of the problem.

Brake fluid should only be kept for 12 months once the bottle is opened. Have you topped up your system with brake fluid since the fluid was changed and if so was it old or new?

Did you do the fluid change/flush yourself or did you have a brake technician do it for you? If it was a technician, was it new fluid from a sealed drum?

It sound to me as though you have water in the system that has boiled due to the high temperature of your brakes. Brake fluid and water are incompressable, however, steam is compressible and can cause you braking to be ineffective. I would suggest that you undertake a completed flush with known new brake fluid (no water present), as you stopping power is the most important safety system you have in your car (in my opinion).

If you really are concerned then you may consider a complete brake system overhaul or follow Mrmerlin's advice?

Tails 1990 928S4 Auto

 

I must admit, that I have not changed the fluid since I own the car and it has been a couple of years. So this fluid may be very old...

 

Brake fluid was my first thought as well. I wonder how it could boil the fluid sitting in traffic.
I tried to heat up the brakes on the highway today to test if it may be the fluid, but I could not get it to replicate the problem.

I will flush the system first thing tomorrow and see how it goes.

 

Sab,
Recommended brake fluid change on S4 is every 2 years. On the later GTS it is 3 year, however, with a better DOT fluid.

Change the fluid, follow the order of bleeding as per the WSM and hope that you don't have any internal corrosion from the water ingress.

Tails 1990 928S4 Auto