Air Brake System Parts: Compressor, Governor, Tanks, Spring Brakes, and Dual Systems

Three braking systems that share one air supply

The model manual describes air brakes as three systems in one. The service brakes apply and release when you use the brake pedal in normal driving. The parking brakes apply and release with the parking brake control. The emergency brakes use parts of the other two systems to stop the vehicle if the brake system fails. All three draw on the same compressed air, which is why a leak or a low reading can affect all of them at once.

This page follows Section 5.1 of Washington's CDL guide, a public copy of the AAMVA model manual that state CDL manuals are built on, checked against California's handbook. The pressures given are the manual's typical values. Your vehicle's maker sets the real ones.

Follow the air: from the compressor to the brake shoe

1. The compressor pumps air into the storage tanks. The engine drives it through gears or a V-belt. It may be cooled by air or by the engine's cooling system, and it may have its own oil supply or use engine oil. If it has its own oil, check the level before driving.
2. The governor decides when the compressor pumps. At cut-out, around 125 psi in the manual, it stops the pumping. At cut-in, around 100 psi, it starts the pumping again. The federal brake standard for new vehicles sets cut-in at 100 psi or more for trucks and 85 psi or more for buses (49 CFR 571.121, S5.1.1.1).
3. The first tank has a safety relief valve, usually set to open at 150 psi, so the system cannot be over-pressurized. If it releases air, something is wrong and a mechanic should find it.
4. The storage tanks hold enough air for several brake applications even if the compressor stops. Each has a drain valve at the bottom.
5. The supply pressure gauges show the tank pressure, with one gauge (or one needle) for each half of a dual system.
6. The brake pedal, also called the foot valve or treadle valve, sends air to the brakes. Pressing harder sends more air. Releasing the pedal lets air out of the system, and the compressor has to replace it, so pumping the pedal for no reason can use air faster than the compressor makes it.
7. The brake chambers at each wheel take that air and push a rod out. The rod moves the slack adjuster, which twists the brake camshaft, and the S-shaped cam on its end spreads the brake shoes against the inside of the drum. That friction slows the wheel, and it also makes heat.
8. The stop light switch, an electric switch worked by air pressure, turns on the brake lights when you apply the brakes.

S-cam drum brakes are the most common type. Wedge brakes and air disc brakes also exist but are less common. Once you can follow the air from start to finish, the 70 Air Brakes flashcards planned for full CDL prep are one way to keep each part's job fresh.

Part by part: what it does and what failure looks like

The governor row deserves a closer look. If the gauge keeps climbing past the normal cut-out, the compressor is still pumping when it should have stopped, and the part that tells it to stop is the governor. The free 20-question CDL set has a governor item built on that clue: answer the compressor governor item in the free CDL questions and check the explanation for each option.

Brake adjustment gets its own federal rules. Air-braked commercial vehicles built on or after October 20, 1994, must have automatic brake adjusters (49 CFR 393.53(b)), and the manual says out-of-adjustment brakes are the most common problem found in roadside inspections. Inspectors judge adjustment by pushrod stroke, against limits in 49 CFR 393.47(e) that depend on the chamber type and size. For a common Type 30 clamp-type chamber, the limit is 2 inches for a standard-stroke chamber and 2½ inches for a long-stroke one. The by-hand slack check in the manual is a quick screen, not that measurement.

Federal rules require ABS on truck tractors built on or after March 1, 1997, on single-unit trucks and buses built on or after March 1, 1998 (49 CFR 571.121, S5.1.6.1), and on trailers and converter dollies built on or after March 1, 1998 (S5.2.3.1). The manual adds that ABS does not change your normal braking power or necessarily shorten your stop, but it helps you keep control.

Dual air brake systems: primary and secondary

Most heavy vehicles have a dual air brake system: two separate systems, each with its own tanks and lines, sharing one set of controls. One system, usually called primary, typically works the brakes on the rear axle or axles. The other, the secondary system, works the front axle and possibly one rear axle. Both supply a trailer if there is one.

Before driving, the manual says to let the compressor build at least 100 psi in both systems. In Washington's guide, the warning light and buzzer should shut off once both systems rise above a value the manufacturer sets, which must be above 60 psi, and should come back on before either system drops below 60 psi; California's handbook gives 55 psi for both. If one system runs very low while you drive, either the front or the rear brakes will not work fully, so your stops get longer. Stop safely and have the system repaired. Questions on dual systems usually turn on which brakes you lose, and full CDL prep planned for launch explains every Air Brakes answer, right or wrong.

Contaminated air: water, oil, and dirt

Contaminated air is its own federal Air Brakes knowledge area (49 CFR 383.111(b)(2)). Compressed air carries some water and some compressor oil, both bad for the air brake system, and both settle at the bottom of the tanks. Manual drains open with a quarter turn or a pull on a cable, and the manual says to drain those tanks completely at the end of each day of driving. Automatic drains expel water and oil on their own, some have heaters so they do not freeze, and some can also be drained by hand.

Dirt is the third contaminant. On a combination, it can get in through the glad hands, the couplings for the trailer lines, which is why the manual says to clean the couplers before connecting and to use dummy couplers when the lines are not in use. The free CDL set includes a cold-morning tank drain item. After you can explain what water does to the system and why an evaporator does not replace draining, try the air tank drain item in the free mixed CDL set.

What a severed or disconnected line does

Severed or disconnected lines between a power unit and its trailer are federal Air Brakes area 3 (383.111(b)(3)). On a single vehicle, a leak that drains all the air will make the spring brakes come on, which is the fail-safe. On a combination, losing the trailer emergency line applies the trailer's emergency brakes and closes the tractor protection valve, which protects the tractor's air. A leak in the service line may not show until you brake. The trailer air brakes guide covers both cases.

To see how each part is tested in the pre-trip inspection, go to the air brake check. For the typical numbers and the full topic list, go to the Air Brakes test guide. When you want more practice than the free set offers, compare the 30-, 90- and 180-day CDL prep options planned for launch.