The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level around 125 psi, the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure around 100 psi, the governor allows the compressor to start pumping again.

When the low air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away, while you can still control the brakes.

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system.

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the "controlled braking" method or the "stab braking" method.

Low air warning devices (buzzer, light, flag) should activate before air pressure drops below 60 psi or level specified by the manufacturer.

Wedge brakes and disc brakes are less common than s-cam brakes.

Snub braking method is used for steep downhill grades, in this method apply the brakes just hard enough to feel a definite slowdown. When your speed has been reduced to approximately five mph below your "safe" speed, release the brakes. When your speed has increased to your "safe" speed, repeat.

In some vehicles, a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring loaded, so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way, so you can control the spring brakes if the service brakes fail.

Without ABS you still have normal brake functions. Drive and brake as you always have.

When you drive a vehicle with ABS, you should brake as you always have. In other words: use only the braking force necessary to stop safely and stay in control; brake the same way regardless of whether you have ABS on the bus, tractor, the trailer, or both; as you slow down monitor your tractor and trailer and back off the brakes if it is safe to do so to stay in control.

Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems, which use a single set of brake controls. One system or primary system typically operates the regular brakes on the rear axle or axles. The other or secondary system operates the regular brakes on the front axle and possibly one rear axle.

The Department of Transportation requires that ABS be on: air brake vehicles built on or after March 1, 1998.

Spring brakes are an important backup system: powerful springs that automatically apply the brakes if they sense that air pressure has been lost for some reason, such as a leak. They will also apply the brakes if the psi gets too low, although ideally, you should take control of your brakes before that happens.

The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.

Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor. If you keep pressing and releasing the brake pedal, air may leave the system faster than the air compressor can replenish it, and the air pressure may drop to the point that the brakes won't work.

Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure.

As the overheated drums expand, the brake shoes and linings have to move farther to contact the drums, and the force of this contact is reduced. So, you will be pressing harder on the pedal.

Converter dollies built on or after March 1, 1998, are required to have antilock brakes. These dollies will have a yellow lamp on the left side of the dolly.

Emergency braking does not mean pushing down on the brake pedal as hard as you can. That will only keep the wheels locked up and cause a skid. If the wheels are skidding, you cannot control the vehicle.

You apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheels lock, release the brakes. Re-apply the brakes as soon as you can.

Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure.

The S-cam is S-shaped. The s-cam forces the brake shoes away from one another and presses them against the inside of the brake drum.

Pull forward at 5 mph, apply the service brake and stop. Check to see that the vehicle does not pull to either side and that it stops when brake is applied.

Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.