brake servo leaking?

[EssexNovaMad1.6]

a mate was playing about with the rear brakes and since then as i brake theres a hissing noise coming from the servo on the inside of the car. this happens all the time. does any1 know if theres a seal on the inside of the car and could playing about with the rear brakes have any relation to the problem?
cheers.

[Mark]

He probably pushed the Pistons on the rear brake cylinder in too fast and blew the servo seal.