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.

The use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear you should only have to apply the brakes just hard enough to feel a definite slowdown.

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

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.

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

Shut the engine off when you have enough air pressure so that the low-pressure warning signal is not on. Turn the electrical power on and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank.

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.

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.

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 put on the brakes by pushing down the brake pedal. Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes.

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

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

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).

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

Drum brakes are on each of your vehicle's axles and contain about 10 different parts, including the axle, slack adjuster, and brake drum itself. While a safety valve is part of the air brake system, it is not part of the drum brake.

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.

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

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

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.

All vehicles with air brakes have a pressure gauge connected to the air tank, and it tells you how much pressure is in the air tanks.

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 air brakes there is an added delay called Brake Lag. This is the time required for the brakes to work after the brake pedal is pushed.

The tractor protection valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak, and it will close automatically if air pressure is low (typically 20-45 psi).

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.

Slack adjusters are an important part of your air brake system that allows you to adjust the brakes to ensure that they are safe. They are located in different places, depending on the type of brakes that you have.

Any time you park use the parking brakes. Pull the parking brake control knob out to apply the parking brakes, push it in to release.

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.

A low air pressure warning signal is required on vehicles with air brakes.

To check slack adjusters on S-cam Brakes. Park on level ground and chock the wheels to prevent the vehicle from moving and apply the parking brake.

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.

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.

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.

When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.

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.

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.

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.

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

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.

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

Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage. Exceptions are when the brakes are very hot, or wet and it is very cold outside. In this case use wheel chocks.

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

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.

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.

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.

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.

If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible.

Even if the low-pressure alarm is for the secondary system you need to stop at a safe location, park, and get the issue fixed. It is unsafe for the vehicle to remain in motion with a low air alarm.

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.

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.

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.