Emergency Air Brake system of a tractor-trailer

There are usually two lines to the brakes. One for normal use, one for emergency use. The piston for the brakes has two sections. Below is normal use and the upper section is seperate and having it's own air supply. If both lines fail and there is no air supplied the brakes will lock up.

The brake can on a semi is just a large diameter, short stroke air cylinder with an internal coil spring that forces it to extend and apply the brakes in the absence of air pressure.

So, theory of operation is this:

When in "PARK" there is no air supplied to the cans; brakes applied.

When "PARK" is released, air is supplied to the rod side of the can, thereby compressing the spring and releasing the brake.

When the driver steps on the brake pedal, air is supplied to the spring side of the can, thereby overcoming the pressure on the rod side and applying the brake. These forces can be balanced very precisely, preserving the "feel" of the brakes.

As you can see, any loss of air pressure results in a "fail-safe" condition (brakes applied).