I have a Renault scenic that has a hissing brake ! you can be drive along normally when the a hissing will start (from the pedal) if you tap the brake then the hissing stops. This also sometimes happens just after you have braked. I recently took the car in to the garage for some other work and explained the problem and asked them to look at the brakes, they couldn't find a problem and said that the brakes were working fine but and typically the brake didn't make the notice on that occasion ! Has anybody got any idea's at it seems to be happening more regularly and is really getting on my nerves !
the rubber daiphram on the brake servo is split. needs a new brake servo. watch as this can be dangerous if it packs up altogether as you will have no vacuum assistance with the brakes. (like braking without engine running, solid pedal.)