Brake Booster

[Black340]

I notice that some times when  I depress the brake pedal I hear  hissing noise that seems to come from under the dash..is that the sign of a power booster punctucre diaphragm ..is any way to check it before buy a new booster?

[GoodysGotaCuda]

Sounds like it'd be a booster issue. there are a couple ways to test a booster in the car.

#1, Engine off, depress the pedal a few times until it becomes hard. Hold the pedal down, start the engine. Good operating booster will let your foot travel down slightly once the engine vacuum hits it.

#2, Let it run. Turn off the engine without messing with the brake pedal. Head back in 15 mins or so, if you have a good 2-3 pumps with vacuum assist, your booster is holding. If its rock solid, you got yourself a leaky booster. . . Or check valve

My dads 02 BMW actually was making a hissing sound depending on engine vacuum, louder at idle and decel, replaced the booster and no more.

[Bullitt-]

A bit of a shooosh is normal especially when the motor is not running, it may make some noise when the motor is running but would be drowned out by the exhust.

[Chryco Psycho]

see if the engine runs differently with the hose to the booster removed & plugged if it does the booster has to be leaking internally

[Black340]

Thank you