Air-operated braking systems are used on heavy vehicles. Compressed air, operating on large-diameter diaphragms, provides the large forces at the brake assembly that are needed.
An air compressor pumps air to storage tanks. Driver-controlled valves then direct the compressed air to different wheel units, to operate the friction brakes.
On articulated vehicles, any delays in applying the trailer brakes should be minimized. This is achieved using a relay valve, and a separate reservoir on the trailer.
This arrangement also applies the brakes if the trailer becomes disconnected from the prime-mover.
