CDL Air Brakes Parts of an Air Brake System Summary

An air brake system combines three braking systems: the service brake (used in normal driving when you press the pedal), the parking brake (used when you park), and the emergency brake (uses parts of the service and parking brakes to stop the vehicle if the service brake system fails).

Wrong choices

• Service brakes, engine brakes, and exhaust brakes: Engine and exhaust brakes are speed retarders, not part of the three air brake systems.
• Front brakes, rear brakes, and trailer brakes: These are places where brakes are, not the three braking systems.

The air compressor governor controls when the compressor pumps air into the tanks. When tank pressure is high enough, it stops the pumping; when pressure falls, it starts the pumping again.

Wrong choices

• The safety relief valve: The safety relief valve only opens to let air out if pressure gets too high.
• The foot valve: The foot valve (brake pedal) sends air to the brakes; it does not control the compressor.

When air tank pressure rises to the cut-out level (around 125 psi, psi = pounds per square inch), the governor stops the compressor from pumping air.

Wrong choices

• About 100 psi: About 100 psi is the cut-in level, where pumping starts again.
• About 60 psi: 60 psi is where the low air pressure warning must come on, far below cut-out.

When tank pressure falls to the cut-in level (around 100 psi), the governor lets the compressor start pumping again.

Wrong choices

• About 125 psi: About 125 psi is the cut-out level, where pumping stops.
• About 150 psi: About 150 psi is where the safety relief valve opens.

A safety relief valve is installed in the first tank the compressor pumps air to. It protects the tank and the system from too much pressure. It is usually set to open at 150 psi.

Wrong choices

• 125 psi: About 125 psi is the governor cut-out, where the compressor simply stops pumping.
• 100 psi: About 100 psi is the governor cut-in pressure.

Compressed air usually has some water and some compressor oil in it, and they collect in the bottom of the tanks. This is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. That is why tanks have drain valves.

Wrong choices

• To lower the air pressure before every stop: You do not drain tanks to control pressure while driving.
• To let the alcohol evaporator work: The alcohol evaporator works without draining the tanks.

In newer vehicles with air brakes, you put on the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes (spring brakes) on and push it in to release them.

Wrong choices

• A red, eight-sided knob: A red, eight-sided knob is the trailer air supply control on combination vehicles, not the parking brake.
• A blue lever on the steering column: Blue is not the standard color for the parking brake control.

A low air pressure warning signal is required on vehicles with air brakes. It must come on before the air pressure in the tanks falls below 60 psi (or one half the governor cut-out pressure on older vehicles). The warning is usually a red light, and a buzzer may also come on.

Wrong choices

• 100 psi: 100 psi is about the governor cut-in pressure, not the warning level.
• 20 psi: By 20 psi the spring brakes may already be on; the warning must come on much earlier.

Spring brakes are held back by air pressure. When the air pressure is removed (by the parking control or by a leak), strong springs put the brakes on. That is why they work as parking brakes and as emergency brakes.

Wrong choices

• The brakes release so the wheels roll freely: Spring brakes do the opposite: losing air puts them on.
• Nothing; spring brakes only work when you press the pedal: Spring brakes are applied by springs, not by the brake pedal.

On tractors and straight trucks, the spring brakes come fully on when air pressure drops to a range of 20 to 45 psi (usually 20 to 30 psi). The low air warning comes on earlier (before 60 psi), so you have time to stop safely before the spring brakes lock on.

Wrong choices

• 60 to 80 psi: 60 psi is the low air warning level; the spring brakes come on lower than that.
• 100 to 125 psi: This is the normal working range set by the governor.