Section 5. Air Brakes Flashcards
Air brakes fall into three different breaking systems. What are the three categories of air brakes on heavy vehicles?
Service brakes.
Parking brakes.
Emergency brakes.
Air brakes consist of many different components. What is the air compressor governor?
The governor controls when the air compressor will pump air into the storage tanks. When air tank pressure rises to the “cut out” level around 125 psi, the governor stops the compressor. When tank pressure falls to the “cut in”pressure around 100 psi the governor allows the compressor to start pumping again.
One component of an airbreak system are air tank drains. Why must air tank drains be drained by using the valve at the bottom?
Compressed air usually has some water and some compressor oil in it. This can freeze in cold weather and is generally bad for the air brake system.
One component of an air brake system is a safety valve. What is the purpose of a safety valve and when does it activate?
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
Foundation brakes are used at each wheel, With the most common being the S-cam drum break. How does this break operate?
When the brake pedal is pressed, air enters each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, twisting the brake camshaft. This turns the ask him which forces the brake shoes against the brake drum.
In an air brake system, a low pressure air warning signal is required on vehicles with air brakes. At what psi must a low air pressure warning signal activate?
A warning signal you can see must come on before the air pressure in the tank falls below 60 psi.

All air brake equipped vehicles must have emergency spring brakes and parking brakes. These brakes are held open by air pressure. If air pressure is removed, the springs put on the brakes. At what psi pressure drop will the brakes automatically engage?
Emergency air spring breaks come fully on when air pressure drops to a range of 20 to 45 psi, (typically 20 to 30 psi).
What is a dual air brake system on heavy vehicles?
A dual air brake system has two separate air brake systems, which use a single set of brake controls. One system operates the regular brakes on the rear axle or axles the other system ups operates the regular brakes on the front axle.
Before driving a vehicle with a dual air system, operators must allow time for the air compressor to build up to a minimum of ______ psi in both the primary and secondary systems?
Allow time for the air compressor to build up a minimum of 100 psi pressure in both primary and secondary systems. Pay attention to the low air warning light and buzzer. The warning light in buzzer should shut off when the air pressure in both system rises to a value above 60 psi.
During vehicle inspection, how should slack adjusters on S-cam brakes be checked?
Park on level ground and chalk the wheels. Release the parking brakes so you can move the slack adjusters. Pour hard on each slack adjuster and measure movement. If Adjuster moves more than about 1 inch where the push rod attaches to it, it probably needs adjustment.
When performing a vehicle inspection, how should air compressor governor “cut in” and “cut out” pressures be checked?
Run the engine at fast idle. The air governor should “cut out” the air compressor at a specified pressure. The air pressure on the gauges will stop rising. With the engine idling step on and off the brake to reduce air tank pressure. The compressor should “cut in” and the pressure should begin to rise.
When performing a vehicle inspection, how should the operator test the air leakage rate in the brake system?
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 2 psi in 1minute for single vehicles in less than 3 psi in 1 minute for combination vehicles.
When performing a vehicle inspection, how should the operator check the air loss rate in the brake system?
With full air tanks, shut off the engine, chock wheels, release parking brake. Fully apply the foot brake and hold for one minute. Check the air gauge to see if the air pressure drops more than 3 psi in one minute for a single vehicle or 4 psi in one minute for a combination vehicle.
When performing a vehicle inspection, how should the operator test the low air pressure warning signal in the brake system?
With the engine off and electrical power on, pump the brake pedal to reduce tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi.
During a vehicle inspection, how should the operator check that the spring brakes come on automatically when testing the brake system?
After completing the low pressure warning signal test, continue to pump the brakes until the parking brake pops out. This should occur when the air pressure drops to between 20 and 45 psi
