Hissing Brakes

[hook]

On my 2001 auto avensis when I take foot off the brake I get a hissing sound inside the car. Inaudble under the bonnet. The noise is coming from a grey confluted plastic sleeve which is positioned around the rod which extends from the brake pedal and goes into the servo. The hissing stops as soon as I brake. But starts again when I remove foot. The car stops OK It is irritating me to hell. As is passengers comments such as whats that hissing noise? Are we losing altitude. Anyone's advice who has had or cured this prob would be appreciated.

[gazza1286]

Sounds like the brake vacuum servo unit has a broken/worn inner seal.

If the unit is working correctly the following test would work:

Sit in drivers sear and repeatedly pump the brake pedal. You will notice that the free-play become less and less and after 3/4 strokes the brake will be very firm and cannot be depressed. It should remain like this for a period if there are no leaks. My guess is that your pedal will continue to give or begin to 'soften' within seconds of firming up. Turning on the engine will return the brakes to their normal feel.

The servo unit assists braking and the vacuum is provided by suction via the intake manifold. The vacuum sounds like it has failed and air is being drawn into the unit through the brake push rod seal. Hence the hissing. As the fault develops you may note that extra force will be required to obtain a braking response.

A repair to the servo unit is not normally possible. A trip to the breakers yard will be required. A relatively straight forward repair. 2 1/2 hours labour, maximum.

Good luck