Hissing Brakes

[Joel]

Hi,

 

Trying to bleed the brakes on my Dad's car, and there's air escaping from where the master cylinder joins onto the vacuum servo.

 

I've replaced the rubber O ring, which I guess is supposed to seal the two, but it's still hissing away when you press the brake pedal. Though I noticed on the under side of the master cylinder is a recess, which is part of the mould, and technically makes it impossible to seal the two surfaces together. Therefore the air must be escaping from elsewhere?

 

Anyway, I'm thinking air shouldn't be escaping like this? (It certainly doesn't on my car), and what's the cause/solution etc?

 

Cheers.

[Guest TerryBarry]

I don't think that is unusual - when the servo works air is drawn into the unit to replace the vacuum [ not the most technical description]

I can certainly hear the same sort of hiss on my car - provided the radio is off

Some cars are more prone to this noise - some have a little foam seal ring which acts as a silencer

[Guest chris brown]

It shouldn’t be able to suck air in at the joint as that for one thing will make it run weak if working correctly the vacuum should in the servo for quite some time after the engine is switched off that’s the way I see it anyway

[Joel]

I should add that the engine isn't running yet, so there isn't actually any vacuum at the servo.

 

On my car, which is an identical setup to my Dad's, I can press the brake pedal (with the engine off), and it feels firm and there's no hissing from anywhere.

 

On my Dad's car, we've bled the brakes (there's fluid at all four corners), but the pedal is still soft and can be depressed right to the floor, accompanied by the hissing which is air escaping from the join between servo and master cylinder.

 

?????

 

[Guest craig030774]

joel im having probs with my brakes i got a nice hard peadal but when you started the engine the pedal went straight to the floor i have fitted new pads slave cylinders hoses no improvement i had a drive in the car and put a hole in the sump and since then i also noticed like a squealching noise from the servo,i have just changed all the seals in the master cylinder but not got me sump on yet so i cannot tell you anymore will let u know next week if i have solved it.also used a pressure bleeder so no air in system

[Joel]

Hmmmm.......think I may have found the answer

 

Hi Bloke,

I give you my 100% guarantee that you servo is stuffed!!

There is a seal that is not replaceable inside the servo that has perished. The servo is sealed and you cannot get into it. However, although not naughty but if the MOT man sees it, it is a fail, but run a fillet of silicone around the joint master to servo and smooth it with a wet finger. WARNING !!!! Do not run the engine till the silicone has hardened. He He.

This is perfectly safe but ultimately you need a servo.

Just as a mater of interest, it was also common on the old Montego's and Maestro's, but you could buy a kit for £4 to fix it, I think old henry wants a ton plus for a new one.

All the best

Simon

 

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