British Columbia has many mountainous roads. You must perform a brake pre-hill inspection (brake check) wherever a sign is posted requiring you to do so before a steep hill ahead. (You may be fined for failing to stop to conduct a brake pre-hill inspection.) It’s also wise to perform a pre-hill inspection before every long or steep downgrade.

An air dryer may be installed in the compressor discharge line between the compressor and the first reservoir. Its purpose is to remove water vapour and contaminants from compressor air before the air enters the first reservoir. The first reservoir has usually been the supply or “wet” reservoir, so called because moisture accumulated in that reservoir before the air flowed from there to the service reservoirs. However, today’s air dryer technology is so effective that a “wet” reservoir may not be needed anymore. In a modern Air Dryer Integrated System (ADIS), the air compressor governor and the air dryer are combined into a single unit. Air from the air dryer goes directly into the primary and secondary service (“dry”) reservoirs

A two-way check valve allows compressed air from two different sources to flow through the same air line. The valve will select the source with the higher pressure. In this case, the foot valve and hand valve are connected to two-way check valves that in turn lead to the tractor protection valve, and from there to the control (service) line. This arrangement allows you to send compressed air to the trailer brakes by activating either the hand valve or the foot valve. (The foot valve will also send compressed air to the tractor brakes.)

For parking and emergency brakes, some buses and motor coaches use DD3 safety actuators in place of spring brakes. DD3 brake chambers have a way to mechanically lock a brake in the applied position.

Trailers with spring brake priority supply compressed air to the spring brakes (to release them) before the service tank has filled.

When coupling a tractor to a trailer, you must connect two air lines: the service line (also known as the control line or signal line) and the supply line (also known as the emergency line). The service line (usually coloured blue) controls the trailer service brakes. The supply line (usually coloured red) controls the trailer emergency (spring) brakes and supplies air to the trailer air tanks

A typical air compressor can produce air pressures of up to 500 psi (over 3,000 kPa), far higher than required for safe operation of the brake system. Therefore, an air compressor governor is employed to keep tank pressures within a relatively narrow range. When tank pressure rises to the “cut-out” level (typically about 862 kPa or 125 psi), the governor prevents the compressor from pumping more air into the tanks. When the tank pressure falls to the “cut-in” level, the governor allows the compressor to pump air into the tanks again. The cut-in level is typically 138–172 kPa (20–25 psi) below the cut-out level.

Automatic slack adjusters adjust themselves to compensate for excessive pushrod stroke. If you only make normal service brake applications of 40 – 50 psi (276 – 345 kPa), the automatic slack adjusters will maintain adjustment only for those shorter strokes, not for long strokes from full brake applications of 100 psi (690 kPa). Therefore, to keep automatic slack adjusters properly adjusted, make full brake applications regularly. For example, try to fully apply the service brakes every time you stop at a red light or stop sign