my brake pedal hisses gently like a punctured tyre until pressed then it stops, i take my foot of again and it hisses again. its always done it and passed two mot, it can only be heard when the radio is off and gentle revs but im curious, is the servo at the end of the pedal or is it a pushrod or what? any ideas what the hissing is ?

Check the vac pipe on the servo might be leaking