Hissing under braking

[Phil H 4]

Out for a short spin this morning I noted a hissing noise ( like an escape of air ) when the brake peddle was depressed.

The braking does not appear to have been effected although I didn't want to take any chances so returned home to investigate on the drive.

The noise level is constant each time the brake is depressed and only when the engine is running.

No apparent leaks from the brake lines and no bubbles in the master cylinder reservoir.

The noise could not be heard when listening to the servo unit and only whilst in the car- coming from the drivers foot well.

I have recently replaced the servo non-return valve due to a expert advice received via the forum when I had a difficult to diagnose clicking noise in the drivers foot well.

Clamping the hose from the servo to the manifold and the noise stops.

Suggestion as to the cause and remedy would be much appreciated.

Many thanks

Phil

[Steves_TR6]

Hi Phil

Do you have slotted/drilled disks?

i have these and they do make this sort of noise

steve

 

[Phil H 4]

Hi Steve,

No to the discs, perhaps an upgrade in the future.

The noise happens when the peddle is depressed the car is stationary with the engine running.

Phil

[Waldi]

Is the noise just for a short period or continuous if you press the pedal?

If it is continuous, it probably is a failed diaphragm. Take the hose off the throttle body and do a vacuum test ( suck it). This will confirm or eliminate this as a cause, but since your brakes work ok, this is probably not the case.

If not: there is a small “filter” in the servo at the brake-pedal side, and a rubber bellow (gaiter); this could be loose; every time the brake is applied, the vacuum pulls the diaphragm to the front, so some air is sucked in from the cabin side. 

Waldi

[Phil H 4]

Hi Waldi,

 

Thanks for the suggestions.

The noise is quite loud initially and then dies out.

I will investigate in the morning and report back.

Many thanks.

Phil

[Phil H 4]

Update.

Sucked on the hose as suggested a little difficult to ascertain as my lungs may not be as good as they need to be.

Blowing confirmed a restriction if this helps.

There is a slight noise - escaping air brake pedal side when the brake pedal is depressed when the engine is off, but much more pronounced when the engine is running, a loud hiss. This continued while the brake is depressed but reduces in volume.

If its a leak from the diaphragm would the brakes still work but with reduced efficiency, and is it an indication that its the diaphragm that clamping the hose from the servo to the manifold and the noise stops.

Thanks for your time and direction.

[Waldi]

Hi Phil,

when I sucked on my hose (sorry for bad language), I could achieve a “sort of” vacuum, but not convincing to be honest. 

I have not really driven the car, still doing bodywork etc. So cannot confirm how well my car brakes with this.

I guess a minor internal leak is not a big issue, as the engine vacuum will compensate for that, to some extend, but am not a servo “expert”.

When I opened my servo, I was surprised how simple it was. I had ordered a small overhaul kit, but not the diaphragm, as this looked still ok. Some parts in the kit fitted, others not. To open it, I made a bracket to open the servo. Basically a large spanner with holes for the 2 studs from the MC. Large is 1 m length or so. The servo I clamped in a vice with the square aluminium adapter plate I think.

Sorry, cannot provide more guidance. Anyone?

Waldi

[Phil H 4]

Hi Waldi,

Thanks for your input, going to take her for a spin if and when the rain stops and see what the brakes are like, nothing downhill !

Will then investigate further and post if I get to the bottom of it.

Thanks

Phil

[John McCormack]

I had the same symptoms. I bought a new servo from Rimmers, problem solved.