Brake drums or discs must not have cracks longer than one half the width of the friction area.

Park on level ground and chock the wheels to prevent the vehicle from moving, release the parking brakes so you can move the slack adjusters, using gloves and pull hard on each slack adjuster that you can reach, and if a slack adjuster moves more than about one inch where the push rod attaches to it then adjustments are needed.

With a fully-charged air system typically 125 psi, turn off the engine, release the parking brake; and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicles brakes released and three with the brakes applied, and less than three psi in one minute for combination vehicles with the brakes released and four with the brakes applied.

When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes.

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.

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 use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine.

With a fully-charged air system, turn off the engine, release the parking brake, and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicle brakes released, and three with the brakes applied.

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

With a fully-charged air system, turn off the engine, release the parking brake, and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicle brakes released, and three with the brakes applied.

Installed on the compressor side of the air tank is a one-way check valve, it allows air into the tanks, but does not allow it to flow back to the compressor.

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.

If the warning signal doesn't work, you could lose air pressure and you would not know it. This could cause sudden emergency braking in a single-circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

The total minimum distance your vehicle has traveled, in ideal conditions; with everything considered, including perception distance, reaction distance and braking distance, until you can bring your vehicle to a complete stop. At 55 mph, your vehicle will travel about 450 feet.

Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect, they are also affected by adjustment, and low air pressure.

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.

To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle.

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.

Increasing application pressure to hold the same speed means the brakes are fading.

Don't use the parking brakes if the brakes are very hot (from just having come down a steep grade), or if the brakes are very wet in freezing temperatures. If they are used while they are very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks instead.

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.

A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on.

Check the alcohol container and fill up as necessary, every day during cold weather.

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.

For manual slack adjustors, the brake pushrod should not move more than one inch, with the brakes released, when pulled by hand.

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.

Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely.

A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle.

Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.

The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brakes system fixed.

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.

Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi).

Application Pressure Gauge shows how much air pressure you are applying to the brakes.

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.

Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.

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.

When main air pressure is lost, the spring brakes come on. Some vehicles have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency.

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.