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.

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Daily air tank drainage is still needed to get rid of water and oil.

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.

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi.

With air brakes there is an added delay or Brake Lag. This is the time required for the brakes to work after the brake pedal is pushed. With hydraulic brake, the brakes work instantly. With air brakes it takes a little time, one half second or more, for the air to flow through the lines to the brakes.

Parking or emergency brakes must be held on by mechanical force because air pressure can eventually leak away. Spring brakes are usually used to meet these needs. When driving, powerful springs are held back by air pressure.

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

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.

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

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.

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

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.

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

When you push the brake pedal, air is let into each brake chamber.

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.

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.

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.

When the engine is at operating rpms, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe.

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.

Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect. Brake fade is also affected by adjustment.

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.

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.

Front wheel brakes have been shown to be ideal under all weather and driving conditions. Front wheel braking is unlikely to cause a skid even on icy roads.

When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the s-cam forcing the brake shoes away from one another and presses them against the inside of the brake drum.

With a fully-charged air system (typically 125 psi), turn off the engine, release the parking brake (push in); and time the air pressure drop.

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.

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.

ABS is a computerized system that keeps your wheels from locking up during hard brake applications.

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

All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force because air pressure can eventually leak away. Spring brakes are usually used to meet these needs.

On both tractor-trailers and single vehicles, to test whether the spring brakes will come on automatically, use the same method you use for testing the low air pressure warning signal: Step on and off the brake pedal, this time until you reach an even lower psi reading and the parking brake valve closes or pops out.

Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction, and also causing expansion of the brake drums.

Stab braking means that you: apply your brakes all the way, and release brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again.

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.

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.

If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring an emergency stop. Don't drive until you get the problem fixed.

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.

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

Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

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.

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.

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.

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

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

The air brake lag distance at 55 mph on dry pavement adds about 32 feet.

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles, but the brakes must be well maintained and used properly.

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.

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.

The service brake system applies and releases the brakes when you use the brake pedal during normal driving.