*** SAFETY ALERT *** " HEARING AIR WHEN YOU APPLY THE BRAKE? " If you hear air when braking, it is NOT a good thing... It indicates that there is air in your brake fluid lines. Have someone bleed the air out of your brake lines as soon as possible. A basic explanation: There is a set of liquid-filled tubes (brake lines) going from near your brake pedal to each of your wheels. By pressing on your brake pedal you force liquid down these tubes, forcing a brake pad against each wheel and stopping your car. As long there are no bubbles of air in any of the tubes then this works great... Brake fluid with no air in it is firm, meaining that any movement at one end of a tube causes the same amount of movement of fluid at the other end. So pressing firmly on your brake pedal makes the brake pads press firmly against the wheels, safely stopping your vehicle. However, if there is any air in a brake line, then part of the movement of your brake pedal is used up trying to squeeze the brake fluid to make it firm enough to move the brakes pads. The wheezing noise - the "air" you hear - is the compression of those bubbles, a sure sign of this problem. The process that removes these bubbles is called 'bleeding' your brakes. ...You do have four tubes, one for each of the four wheels, so having one bubbly tube might still allow you to stop the car - but not very safely and probably not in a straight line since one wheel is not going to brake as well as the others do. To fix this, a mechanic will open a brake line and put brake fluid into the top end while another mechanic 'bleeds' the bubbles out of the other end of it, creating an air-free column of liquid in the line. They then close both ends of the line again so that it does not leak or get air in it again. When all tubes are air-free, you stop better and there is not any 'softness' or wheezing of 'air' when you press the brake pedal. For the your safety, please do this as soon as possible.
More info, please. Namely, are you talking about an air brake system? You should hear some air as you apply and release the brake, but you shouldn't hear a continuous air sound as the brake is applied - if so, you've probably got a compromised air line or a bad brake chamber.
The power brake booster is leaking vaccume.
If you're referring to an air brake system, you have a leak in a brake chamber... one of your emergency chambers has gone bad (if you hear it only when you depress the brake pedal, then it's a service chamber).
Yes
Instead of using hydraulic brake fluid to apply the pads to the brake drums or discs, air pressure is used.
In all cars and most trucks HYDRAULIC pressure is used to apply the brakes. In an air brake system such as is found on medium and heavy duty trucks the air is actually released to apply the brakes.
Air unlike brake fluid can be compressed. When there is air in the brake system it compresses when you apply the brakes. This causes a loss of brake pressure on the brake pads and results in much longer stopping distances. This is dangerous and will cause an accident.
Air wedge brakes apply air in a chamber pushing a wedge between two rollers, connected to one end of a brake shoe, causing them to split apart, in turn causing the two shoes to be forced into the brake drum.
Assuming you're in an air brake equipped vehicle, it could be a number of reasons. Loose air line fittings, ruptured air lines, or bad gaskets in one or more brake chambers would be the primary culprits.
Assuming you're talking about an air brake system, if this is happening, then you have either a bad service chamber in at least one of your brake chambers, or a compromised supply air line. What to do should be obvious - get it fixed.
Sounds like your power brake booster diaphragm is busted or there is a leak in the vacuum line going to the brake booster.
If you are driving and you hear a high pitch squeeling/screaching sound but when you apply the brakes the sound stops your brakes need replacing, this is called your brake squeeler.