Technical air release sound when press brakes down

[puntonew]

Another issue! with this car is when i press the brake pedal, it has that air release kind of sound going on.
Like when you press the pedal, air is being released somewhere.
Like the noise an airbed pump makes that you tread on.

I had the same noise on an astra and vectra.

all 3 cars brakes work/ed fine.

What is this noise?

also, the brake pedal is not the most solid feeling, in terms of where it applies pressure to brakes once pressed down, and it is possibly creeping down a little under pressure....

 

[fingers99]

Noise will be from the servo. Pedal going down may indicate that the fluid needs changing or a leak of some description.

 

[a1ant]

Hi mate,

As above, the hiss noise is a perfectly normal function of the brake servo.

The pedal should not continue to creep all the way to the floor. If it creeps a small amount, its fairly normal, so depends how much a "little" is.

If it simply creeps down a bit then stops, it may be air that needs bleeding from the system.

If it continues to creep down then you may have a leak, this could be an external leak (ie your brake fluid level will be going down, and you should see signs around the leak area) Or internal past one of the seals in the master cylinder.

Alan

 

[puntonew]

The pedal should not continue to creep all the way to the floor. If it creeps a small amount, its fairly normal, so depends how much a "little" is.

thanks mate

yes it is probably a small amount, as i was on a hill yesterday, braked with the engine running, and kept repressing the brake pedal after it had reached the bottom, and each time it seemed to go down a little further, but i didnt do it for long enough to see if it eventually met the floor.

....and only when the engine is running, when the engine is switched off (and car stationary), the brake is solid and at the top.

 

[atom0007]

Engine off = no vacuum assist

That's why it's hard afaik

 

[benny279]

Mine makes this noise, and its perfectly normal and my brakes work great as fitted new pads a week ago I think.

Did quick stop tests this week in a deserted area and well.....lets say I wouldnt want to have a full stomach if I had to stop the car in a hurry for whatever reason

 

[Ziggy122]

Engine off = no vacuum assist

That's why it's hard afaik

Yup The vacuum has been depleated and you get a dead pedal, some stomping power is required here

A Servo check is quiet simple

Engine off - pump pedal, it will get harder with what then seems little pedal travel
Hold the pedal down, then start engine, the pedal should then sink about another bit and sorta stop

If your straight to bottom the servo may forwelll be working, its probably air in the brake fluid at fault here, as the assistance given easily squish's any air in the line

Ziggy

 

[portland_bill]

Spot on Ziggy.

The servo works on a simple principle. The big black can has a diaphragm in it. When the engine is running, vacuum (from the inlet manifold on a petrol, or a vac pump on diesels and often also on turbo petrols) is applied to the servo. This is allowed to apply to both sides of the diaphragm, so no effect. When the brake is applied, a valve shuts off the vacuum from the driver's side of the diaphragm and allows atmospheric air in. This is the sound you can hear inside the car as the air passes the pedal pushrod. (There's usually a filter in there, just a bit of sponge.) The atmospheric pressure on one side of the diaphragm will help push the brake.

With the engine off, the remaining vacuum will usually allow 3 applications of the brake before it is exhausted, allowing you to stop safely if the engine or vac pump fails. Once the vacuum is exhausted, you have no help with the braking, hence the solid feel to the pedal. Stopping the car now will take some push.

As Ziggy has described above, the test is when the engine starts. The pedal sinks a little as the vacuum pulls on the engine side of the diaphragm.

Any questions? There'll be a test later.