Q: I change the servo unit with another one but I still hear a sound like a "ssssssssssssss" for two or three seconds after I am turning the engine off, from the brake system. The servo unit is working because I feel difference when I am pushing the pedal when the engine is on and when the engine is off. Inside the brake master cylinder there is a little seal which I change trying to fix the problem and for two or three days after this change there was no sound but now the sound came again. But I don't have any problem with the brakes and because of the difficulty to find another brake master cylinder , do you think that I will have safety problems in the future?

A: Have you checked the vacuum hose from the engine to the brake servo?
Is the servo one way valve (at the end of this hose) OK?
It is possible that the large rubber seal (O-ring type) between the master cylinder and the servo is poor although I don't think it is likely. The servo presumably is OK since you replaced it. You did not say if the servo can retain negative pressure after the engine is switched off. In fact, the brake pedal should become hard after pumping it a couple of times and the engine is off for one hour or so.

If the master cylinder is in bad condition you will have brake failure (brake pedal hitting the floor) which is not rare if the brake fluid is not regularly replaced. In fact, the master cylinder can badly corrode inside due to the moisture built up in the old brake fluid. If you are worried about safety and if you know how to do it, replace all the rubber seals in the brake master cylinder and check that its internal components are not damaged by corrosion. Master cylinder seals are readily available through specialist suppliers. When I bought my Fulvias I did overhaul the master cylinders of both cars immediately.
- Andrea -