Thread: My brake pedal makes a "hiss" when I push it...is this normal?

Originally Posted by dullbird

Same

That's normal operation, may be slightly greater if your hydraulics are not 100% bled, or shoes out of adjustment on drums, giving more pedal travel. Booster leaking makes a constant hiss, causes bad idling on a V8

Originally Posted by bee utey

That's normal operation, may be slightly greater if your hydraulics are not 100% bled, or shoes out of adjustment on drums, giving more pedal travel. Booster leaking makes a constant hiss, causes bad idling on a V8

Given a leaking booster gives a constant hiss, what causes a hiss when you press the brake pedal?

thanks

Originally Posted by Mick_Marsh

Given a leaking booster gives a constant hiss, what causes a hiss when you press the brake pedal?

thanks

When your brake booster is connected to a vacuum source (V8, vac pump) the air is vacced out of both sides of the booster creating no net force. When you put your hoof on the brake pedal a valve in the cabin end of the booster lets air into the back side of the booster. That is what makes the booster a booster, the air pressure at the back is pushing forwards towards the vacuum. When you take your hoof off, this valve closes, inside another opens and vacuum is applied to both sides of the booster again. A spring helps return your pedal to the "off" position.
So you are hearing air enter the booster through a little air filter under the dash, around where the brake pedal push rod goes into it.

Ahhh well that makes more sense then, just wanted to make sure. I didn't think it was a dud brake booster because I know what that's like...my old Falcon's brake booster went and I was driving with half dead brakes up in Queensland for a couple of months But I was careful Fixed it myself and it was good as new.

This I just wanted to check, I thought it would be normal but that's made me feel better about it anyway so cheers

Originally Posted by bee utey

When your brake booster is connected to a vacuum source (V8, vac pump) the air is vacced out of both sides of the booster creating no net force. When you put your hoof on the brake pedal a valve in the cabin end of the booster lets air into the back side of the booster. That is what makes the booster a booster, the air pressure at the back is pushing forwards towards the vacuum. When you take your hoof off, this valve closes, inside another opens and vacuum is applied to both sides of the booster again. A spring helps return your pedal to the "off" position.
So you are hearing air enter the booster through a little air filter under the dash, around where the brake pedal push rod goes into it.

So, given that there is a hiss whilst the brake pedal is depressed and it doesn't stop until I release the brake, the diaphragm may be ruptured. If this is the case, It should be affecting the idle as well. Am I right?

Originally Posted by Mick_Marsh

So, given that there is a hiss whilst the brake pedal is depressed and it doesn't stop until I release the brake, the diaphragm may be ruptured. If this is the case, It should be affecting the idle as well. Am I right?

Carby engines usually have the brake vac hose going to no. 8 cylinder, and a continuous air flow from the booster would cause rough idling. On an EFI engine the vac hose goes to the plenum, the effect might only be noticeable as a rise in idling speed.

A more effective test is to get someone in the car to push the brake pedal while you are under the bonnet, engine idling. Squeeze shut the vac hose to the booster with flat pliers, if this causes the pedal to push up and the hissing stop then you are definitely looking at a crook booster.