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

[Young Angus]

Sitting in traffic in my disco I notice that when I push my brake pedal I hear a hiss as I push harder. Is this noise normal or is there pressure getting out somewhere where it shouldn't be? Apologies if this is a silly question...

[bee utey]

Sitting in traffic in my disco I notice that when I push my brake pedal I hear a hiss as I push harder. Is this noise normal or is there pressure getting out somewhere where it shouldn't be? Apologies if this is a silly question...
Probably the brake booster is dying, it sucks continually from behind the brake pedal, if it gets worse, soon you might have no power assistance at all.:(

[Mick_Marsh]

I'm getting a similar problem with my 101.
I've tracked it down to air being sucked into the brake power booster.
It wasn't doing this until I reco'd the master cylinder.
I'll be watching this thread with interest.

regards
Mick

[roverspec]

Mine did that about a year ago. It was a dead brake booster and the end i had such a hard brake pedal, it was getting dangerous.

It was still origanal so i got my value out of it after 12years.

Put a new one on and right as rain.

A major point if you are doing it yourself there is a spacer on your brake booster you might miss.REMOVE it and put it on the new one as this sets the pressure the brake master, if you dont ttake it off you can drive down the road and your brakes will start to lock on it can get messy.

And PS Mick your sound like the brake booster as well, but with yours you can get it rebuilt as for the disco it is not rebuildable.

Good luck
carla

[dullbird]

mine has hissed on the car for years...I'm sure I asked about this before and told it was fine :eek: as I think I asked about the pedal sinking too after pumping it up

However I will now look into replacing it if that is not the case.

[Young Angus]

Hmm it might be normal...hey do the discos have abs as standard or would mine not have it?

[dullbird]

mine has ABS so the hissing is not just to the non abs

[bee utey]

Hmm it might be normal...hey do the discos have abs as standard or would mine not have it?
Disco ABS was optional, basic system has a brake booster with master cylinder, ABS is on the LH mudguard on the TDi I have here.

[Mick_Marsh]

And PS Mick your sound like the brake booster as well, but with yours you can get it rebuilt

Good luck
carla

I think mine might be simpler as it was so sudden.

Timeline: No problems > master cylinder leaks > I remove master cylinder > master cylinder rebuilt > I reinstall Master cylinder with lots of prodding, poking and head scratching > hissing noise.

I've done something. I need to investigate further but at the moment, I'm time poor.

[setsuna]

Mine makes that 'hiss' but its not constant - its only as the pedal is moving downwards.
has been doing it for perhaps a year - no other bad effects noticed (yet).

David.

[dullbird]

Mine makes that 'hiss' but its not constant - its only as the pedal is moving downwards.has been doing it for perhaps a year - no other bad effects noticed (yet).

David.


Same

[bee utey]

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

[Mick_Marsh]

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

[bee utey]

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.

[Young Angus]

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 :o 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 :D

[Mick_Marsh]

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?

[bee utey]

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.

[Young Angus]

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?

What I found when I had a stuffed brake booster in an old car is that when the pedal was pressed at all I would hear a "sucking" sound and yes, the idle would go a bit rough while the pedal was down. The sucking sound would last until I let go of the brake pedal all together. The sucking sound happened because the seal around the brake booster was gone and it was sucking air from outside while it should have been air tight.