MS CDL Air Brakes flashcards
MS CDL Air Brakes · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Governor cut-out pressure
- The pressure at which the governor stops the compressor from pumping, typically about 120 to 125 psi in a properly functioning air brake system.
- Governor cut-in pressure
- The pressure at which the governor restarts the compressor to build pressure back up, typically about 100 psi.
- Safety valve setting
- A valve on the supply tank that releases air at about 150 psi to prevent over-pressurization and tank rupture.
- Slack adjuster function
- A lever that converts pushrod stroke into rotation of the S-cam, spreading the brake shoes against the drum to slow the vehicle.
- Low-pressure warning threshold
- A buzzer and light that must activate before system pressure drops below 60 psi, warning the driver of insufficient air pressure.
- Alcohol evaporator purpose
- A device that adds alcohol to the air system to prevent moisture from freezing in valves and lines in cold weather.
- Primary circuit
- The circuit in a dual air brake system that operates the rear brakes; it is fed from the supply reservoir through a check valve.
- Secondary circuit
- The circuit in a dual air brake system that operates the front brakes; it is separate from the primary circuit so one failure does not disable both.
- One-way check valve
- A valve between the two circuits that allows air to flow one direction only, preventing a leak in one circuit from draining the other.
- Spring brake operation
- Heavy springs apply the parking brake when air pressure is released; they require air pressure to release, making them the failsafe brake.
- Tractor protection valve
- A valve that closes and prevents air loss from the tractor if the trailer breaks away or the trailer air line separates.
- Trailer air supply valve
- The red knob that controls air to the trailer; pulling it out applies the trailer spring brakes and closes the supply line.
- Static leak test limits
- With engine off, brakes released, and pressure at cut-out, loss must not exceed 2 psi per minute for a single vehicle or 3 psi for a combination.
- Applied leak test limits
- With engine off and brakes fully applied, pressure loss must not exceed 3 psi per minute for a single vehicle or 4 psi for a combination.
- Slack adjuster stroke check
- Pushrod stroke must be within the manufacturer's limit; a stroke beyond the limit means the brakes are out of adjustment and the vehicle must not be driven.
- Parking brake test
- With the engine running and pressure built, pull the yellow knob and try to pull forward; the vehicle should not move.
- Service brake test
- Release the parking brake, move forward slowly, and apply the brakes; the vehicle should stop smoothly without pulling to one side.
- Low-pressure warning check
- With the engine off, pump the brakes and confirm the warning buzzer and light activate before pressure falls below 60 psi.
- Leak location method
- Listen for escaping air and apply soapy water to fittings, hoses, and valves; bubbles indicate the leak point.
- Supply-side leak symptom
- The compressor runs constantly or cut-out pressure is not reached, indicating a leak between the compressor and the supply tank.
- Service-side leak symptom
- Pressure drops when the brakes are applied and stays low while applied, indicating a leak between the foot valve and brake chambers.
- Air tank draining
- Drain air tanks daily to remove moisture and oil; a large amount of oil can indicate compressor problems.
- Air dryer purge check
- The air dryer should purge moisture at cut-out; constant purge or moisture in the tanks indicates a dryer problem.
- Brake lag
- The delay between pressing the brake pedal and the brakes actually applying, caused by air traveling through the lines; it increases stopping distance.
- Downgrade braking technique
- Select a low gear before starting down and use the brakes only to control speed, releasing them to cool; do not ride the brakes.
- Pedal pumping effect
- Pumping the brake pedal releases air pressure and can reduce braking effectiveness on a long downgrade; hold the pedal firmly instead.
- Engine braking use
- Downshifting to use engine compression to slow the vehicle on downgrades, reducing reliance on the brakes and preventing overheating.
- Cold weather brake care
- Keep the air system dry and use the alcohol evaporator so moisture does not freeze in valves and cause brake failure.
- Spring brake emergency use
- If service brakes fail, pull the yellow knob to apply the spring brakes gradually; sudden application can lock the wheels and cause a skid.
- Automatic spring brake application
- When system pressure drops to roughly 20 to 45 psi, the spring brakes apply automatically, slowing and stopping the vehicle.
- Downgrade brake failure response
- Downshift to a lower gear for engine braking and apply the parking brake gradually; avoid sudden application that could cause a skid.
- Post-failure requirement
- After any brake failure, do not continue driving until the system has been inspected and repaired by a qualified mechanic.