Air/Hissing Noise from Brake Pedal

there is no air in a hydraulic system or if there is , the clutch will not disengage
if you have any sound when depressing the clutch pedal, it is most likely from the throw out bearing running dry when pressure is applied to the pressure plate fingers
so it amounts to this
if you press on the clutch pedal and you can readily engage first gear or reverse and you have steady operation when you take your foot of the pedal , the clutch system is working as it should
that means that any noise you get when pressing on the pedal is when the throw out bearing is placed under load and as there is no other part that is moving, that is the logical area
one point here --if it is an air noise and of that you are sure , when people press on the clutch pedal, to stop vehicle movement they also have the foot resting or pressing on the brake and a hissing noise here can be from a vacuum leak at the brake pedal from the brake booster
easy to check before pulling a box that doesn't have a problem

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 is probably a vacuum leak in the brake booster. With the pedal going to the floor There is either no brake fluid or you will have to bleed the system. Replace the booster and bleed the brakes starting at the longest line first

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

You need to have the brakes inspected. This noise is the brake pad wear warning tab hitting the brake rotor.;

The air seeping is a dead giveaway. Your power brakes are now manual brakes, but stiffer. The diaphram in the brake booster has torn and the hissing noise is the engine vacuum escaping (you hear it louder when you depress the pedal right?). Replace the booster and you will be fine. The brake lights probably don't work because you can't push the pedal down far enought to release the switch. Please rate. Thanks.

Sounds like your brake booster but can't be sure until I know what kind of truck or car you have....

Hello;
Yoou most likely have a vacuum leak in the line from the booster to the intake. It could be a bad booster or check valve as well.
Thanks;
Rich
RPM Northwest

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.