- Why must air tanks be drained?
- To drain all the air out of the tank
- To drain moisture and oil out of the tank
- To keep anyone from moving the vehicle
Moisture and oil can collect in the air tanks and cause brake failure or corrosion. Regular draining keeps the air system clean and safe.
- What is a supply pressure gauge used for?
- To show how much diesel fuel is in the vehicle
- To show how much water is in the air tank
- To show how much air pressure is in the air tank
The supply (or reservoir) pressure gauge indicates the air pressure available in the tanks. Safe operation requires adequate pressure before driving.
- All vehicles with air brakes must have a low air pressure warning signal.
- True
- False
A warning light or buzzer alerts the driver if air pressure drops too low for safe braking, typically around 60 psi.
- What are spring brakes?
- Brakes used only for parking
- Brakes that apply automatically if air pressure is lost
- The spring under the brake pedal
Spring brakes use mechanical force from large springs to apply pressure if air leaks or fails, providing an automatic safety backup.
- Front wheel brakes are good under all conditions.
- True
- False
Modern braking systems are designed for balanced braking. Disabling front brakes reduces control and increases stopping distance.
- When you apply the brakes, the air brake system uses an electric switch to turn on the brake lights.
- True
- False
The air pressure activates an electric switch that lights the brake lamps, signaling other drivers that you’re slowing down.
- What do air brakes use to make the brakes work?
- Compressed gas
- Compressed oil
- Compressed air
Air brakes rely on compressed air rather than hydraulic fluid. This air pressure activates the brake mechanism at each wheel.
- Air brakes actually consist of three separate braking systems: the service brake, the parking brake, and the emergency brake.
- True
- False
Each air brake system has a specific function: service brakes operate normally, parking brakes hold the vehicle, and emergency brakes engage when pressure fails.
- The service brake applies and releases:
- The parking brake when you use the parking brake control
- The brakes when you press the brake pedal during normal driving
- The brakes only in the event of an emergency
The service brake is the main braking system used in everyday driving. It responds when you press the brake pedal.
- The parking brake applies and releases:
- The brake pedal during normal driving
- The brakes automatically in emergencies
- The parking brakes when you use the parking brake control
Parking brakes hold the vehicle stationary when parked. They are controlled by a hand valve or dash knob, not the foot pedal.
- The emergency brake system:
- Uses parts of the service and parking brake systems to stop the vehicle if the service brakes fail
- Uses only the service brake system
- Uses only the parking brake system
The emergency system combines service and parking brake components to provide braking if there’s a major air loss or system failure.
- The governor controls when the air compressor will pump into the air storage tanks.
- True
- False
The governor starts and stops the compressor to maintain the proper range of air pressure, usually between 100 and 125 psi.
- The air storage tanks are used to hold compressed air.
- True
- False
Air storage tanks store compressed air from the compressor for later use when applying the brakes.
- When you push down the brake pedal:
- Air pressure is reduced to the brake pads
- Air pressure is sent to the brake chambers
- Compressed air is released from the system
Pressing the pedal sends compressed air to each wheel’s brake chamber, forcing the brake shoes against the drum to slow the vehicle.
- The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. It is called the S-cam because:
- It is shaped like the letter Z
- It is shaped like the letter N
- It is shaped like the letter S
The S-cam has a curved “S” shape that twists when the brake chamber pushrod moves, spreading the brake shoes against the drum.
