Brake Problem (brake booster??)
 

Hello all,

I own a '96 A4 2.8, 107k miles and recently there is a problem that mysteriously comes and goes. Whenever there's slow traffic and it's a hot day (above 90 deg) while I'm driving (all 4 times it happened to me on the freeway), suddenly the break pedal gets extremely stiff, the car seems to have a trouble going forward, as if I was pressing the brakes and the gas simultaneously and I also get a smell of burning rubber... (brake pads??) When I manage to stop at the shoulder, turn off the engine for 2 minutes and turn it back on the brakes return to normal and I can drive the car again. The brakes work even when the pedal is stiff (if I press them hard the car stopts). My mechanic (although I don't believe he spent much time examining the car) said he can't see any problems with the brakes. I changed the brake pads 8 months ago, bleeded the whole system, everything was cool. No fault codes on the brakes, it seems this is a brake booster related problem but why it's happening only at high temp conditions? (I don't know that much of car engineering so...) Thank you all for your time,

Theodoros Zanos.

It could be the booster
 

Could also be the vacuum line running from the engine to the booster has sprung a sporadic leak. It runs through the bulkhead before the firewall so its easy to find.....check it for leaks and let us know.

I've had an older Audi with the hydralic booster do that...
 

It was the booster, never seen it happen on a vaccum booster, but I don't think a vaccum leak would bind the brakes.