Hissing Sound from Brake Pedal - Dodgy Servo?

[ChasR]

ATM, one of the cars has a hissing sound coming from the brake pedal almost all of the time when the car is running. As the pedal is pressed it changes into what I can only describe as a squelchier sucking sound with the pedal going pretty much almost all of the the way to the floor before the brakes really start to do anything.

I thought it could mean a knackered servo initially, but surely then the pedal would go hard much earlier on and not really attempt to bury itself in the floor.

Inside the engine bay the hissing cannot be heard at all.

What do you fellas of RR reckon is reason for Darth Vadar hiding beneath my dash? Knackered servo, master cylinder or both?

[scruff]

Master cylinder up the spout?

[akku]

turn the engine off. Pump the brakes until the vacuum is all gone. Compare brake pedal feel.

hissing is usually air, and the servo is vacuum (absence of air if you like) operated.

squelching is more of a fluid thing.

see if you can isolate them into two problems.

[bl1300]

Sounds like servo to me but you may also have hydraulic issues as well.

Look in the footwell around the pedal for signs of fluid and around the master cylinder on the other side of the bulkhead from the pedal for fluid, if you cant see any it doesn't mean there is no leak as it could be filling up the servo.

To test the servo operation pump the brake pedal with the ignition off to expel any residual servo action. It should go quite hard. Then with your foot still on the brake pedal start the engine. As the engine starts a vaccum should build up in the servo and the pedal should sink under your foot.

[bortaf]

You may find the master has leaked into the servo and rotted the diaphram ? does it use brake fluid at all ?
The hissing sounds like satndard servo rear seal leaking, usually when cold then gets better when it warms up? had many a Mk3 transit do that

[barty]

if the pedal goes to the floor with no brakes and your not losing any brake fluid then it sounds like the master cylinder lealing past its seals, if your not sure and its old i would change it anyway

[ChasR]

I am thinking it is the Master after more investigation.

On the back of the master cylinder there is evidence of something going down the front of the servo just below the master cylinder (where they join). I have tried to pump the pedal (no change) but again it goes to nearly the floor before doing anything. That is with the car running and turned off.

However, it doesn't explain why the pedal is hissing alot unless it has taken the servo out with it.

Another point is that the Brake pressure warning light illuminates with the car off, but goes off when it is started. I guess even with a dodgy master Cylinder it would be illuminated?