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.

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

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

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

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.

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.

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

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

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.