Brake servo making a hissing sound around the brake pedal but

The brake servo or booster is divided into two chambers separated by a flexible diaphragm. Briefly there is vacuum on one side and pressing the brake pedal allows air to enter the other side and this is how the brake pedal effort is boosted.

The air enters via a filter through which the pushrod connected to the brake pedal passes. When the brake is applied the hiss of air entering the chamber will be audible; more on some models than others.
When the forces are balanced; when the brake is sufficiently applied no more air will be required and the hiss will cease. When the brakes are at rest; not applied there should be no hissing.

If there is it indicates a problem with the brake booster, perhaps dirt in the valve gear. Although repairs are possible they aren't very practical to carry out. A repair shop would probably charge more for fitting an overhaul kit than for replacing the unit.

The stop lights won't go out until the pedal is depressed, as there is a switch which is activated by the pedal.

A hard brake pedal is probably caused by a defective servo. The hissing noise is probably an air leak in the servo.

Switch the engine on, open the bonnet (the hood..) and see if you can locate the hissing - that'll probably be in the brake servo area.

If you don't know what a brake servo looks like here is a generic image:

it sounds like the brake servo isn't working but you should still have some better brakes than you dascribe so i would also check the master cylinder , whick is connected directly to the servo , quick check ,engine running remove, brake fluid resovoir cap, pump brake pedal and get some to look into the fluid resovoir and tell you is there are air bubbles or not , if bubbles most likely a seal has blown if no bubble then most likely a mechanical part of the brake system ie master push rod at fault , either way i'd get a mechanic to check as you dont take chance's with brakes

Brake servo leaking

I wonder if you have an air leak in the brake servo? Check the manifold vacuum as you operate the brakes. Look for bad hoses around brake servo.

hissing noise is a leaking diaphram inside power brake booster.you have to replace the power brake booster.

this noise is caused by a defective power brake vacuum booster, (big round thing behind the master cylinder) the diaphragm is leaking vacuum.

Sounds like it could well be the brake servo unit.

check that the vacuum line is feeding the servo any leaks in the vacuum line will stop the brakes from working abs relays on the pedal pressure to activate abs, if you can hear a hissing sound from the servo then the servo is leaking inside this is if the vacuum line is not damaged, use a pipe or screwdriver on the case and put to your ear and you should hear if its hissing or not.

Hi,
From your description this appears to be a Brake servo issue.
The brakes are assisted by a vacuum created from the draw off the inlet manifold, it gives assistance too the brake pedal pressure by a large tank/cylinder mounted on the Bulkhead/firewall.
I feel that the sound you a hearing is air being draw into the Brake Servo from the foot pedal seal and the reason for fluctuation in the engine is the vacuum from the inlet manifold is being upset.
Attention I suspect should be addressed to the Brake servo/pedal seal or gaitor.
I hope this clarifies your thoughts?
Please remember to rate my effort.
Paul 'W'
Onyer~EDSon(:0) {#><U.K.