The Bendix Air Brake Handbook
Straight from the source, this comprehensive 63 page pdf download explains everything you wanted to know and more about air brake systems.
From the one-page Introduction to Air Brake Systems:
Air Supply
The vehicle’s compressor takes in filtered air, either at
atmospheric pressure from the outside (or already at
an increased pressure, from the engine turbocharger
in some cases), and compresses it. The compressed
air is delivered to the air dryer where water and a small
amount of oil is removed. The air then travels into the
air reservoirs (”air tanks”) - typically delivered to a
rear brake system reservoir and a front brake system
reservoir as well as any attached trailer reservoirs. For
each system, the air pressurizes the reservoir and the
air hoses all the way to the next control valve, where
the air pressure remains, ready to be used.
A vehicle may use compressed air for many tasks. Some
examples are: to provide force for braking, to deliver
air to a particular component, to off-load bulk goods,
etc.Normal Braking
When the driver applies the foot brake, a plunger within
the foot brake valve moves, opening channels within
the valve that allow the air pressure waiting there to
pass through and be delivered to the rear and front
brake systems. The pressure quickly increases in the
brake chambers and applies force to the push rod,
transferring the force to the S-Cam or air disc brake.
(See page 22 for more about foundation brakes.)
Frictional forces slow the wheels and the vehicle comes
to a stop. When the brakes are released, the air in the
brake chambers is able to be quickly released and enable
the driver to drive away.
