MT CDL Air Brakes flashcards
MT CDL Air Brakes · Build recall with a focused review session.
33 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 unloads the compressor and stops it from building more air, typically around 120 to 125 psi in a normal air brake system.
- Governor cut-in pressure
- The pressure at which the governor reloads the compressor so it resumes building air, typically around 100 psi, roughly 20 to 25 psi below cut-out.
- Supply (wet) reservoir
- The first tank after the air dryer, which collects moisture and oil before air reaches the primary and secondary circuits; it has a drain cock that must be drained daily.
- Safety valve function
- A valve on the supply reservoir that opens near 150 psi to release excess pressure, protecting the system if the governor or compressor unloader fails.
- Slack adjuster role
- A lever that converts pushrod stroke into brake shoe movement and multiplies braking force; it must be checked for free stroke and correct adjustment.
- Type 30 chamber force
- A brake chamber with about 30 square inches of effective piston area; at 100 psi it produces roughly 3,000 pounds of pushrod force.
- Dual circuit purpose
- A split air system with two separate circuits, usually front and rear, so that a failure in one circuit still leaves the other able to stop the vehicle.
- Low pressure warning point
- The warning light and buzzer must activate before system pressure drops below 60 psi, alerting the driver before spring brakes begin to apply.
- Spring brake operation
- Parking brake chambers held applied by heavy springs and released by air pressure; when air pressure is lost, the springs apply the brakes automatically.
- Tractor protection valve
- A valve that closes to keep the tractor air supply from being lost through a trailer breakaway or disconnected line, and controls trailer air supply.
- Static leak test limit
- With the engine off, system fully charged, and brakes released, pressure should not drop more than 3 psi in one minute for a single vehicle.
- Applied leak test limit
- With the brakes fully applied and held, pressure should not drop more than 4 psi in one minute for a single vehicle; a larger drop indicates a leak.
- Pressure buildup test
- With the engine at operating rpm, the system should build from 85 to 100 psi in about 45 seconds for a single vehicle; slower buildup indicates a defect.
- Spring brake test
- With pressure above 100 psi, release the parking brake, then drain pressure and confirm the spring brakes apply automatically as pressure falls.
- Brake lag distance
- The delay before braking force reaches the wheels as air travels through lines and chambers; at 55 mph it can add roughly one vehicle length to stopping distance.
- Brake fade cause
- Loss of braking force when overheated drums and linings lose friction, usually from continuous light brake application on long downgrades.
- Downgrade braking technique
- Select a low gear so engine braking holds speed, then apply brakes firmly and briefly to shed speed and release to let drums cool.
- Skid recovery technique
- Release the brake pedal to let the wheels roll and regain traction, then reapply with less pressure once the vehicle is under control.
- Service brake failure response
- Downshift to use engine braking, apply the parking or spring brakes gradually, and steer toward an escape ramp or safe runoff area.
- Spring brake application caution
- Do not apply spring brakes abruptly at high speed; they can lock the rear wheels and cause a skid, so apply them progressively.
- Trailer breakaway result
- The tractor protection valve and trailer emergency line cause the trailer spring brakes to apply, stopping the trailer if it separates from the tractor.
- Parking brake test
- Apply the parking brake, release the service brakes, and confirm the vehicle does not roll on a slight grade; also check that the valve holds pressure.
- Spring brake caging
- A mechanical bolt used to release a spring brake for towing or when air is unavailable; caging is for moving a disabled vehicle, not a repair.
- Parking brake valve pop-out
- The yellow diamond valve should pop out and apply the spring brakes when air pressure drops below a set point, typically around 45 psi.
- Parking on a grade
- Set the parking brake, place the transmission in gear or park, and chock the wheels; do not rely on air pressure alone to hold the vehicle.
- Compressor oil passage
- Oil entering the air system from worn compressor rings damages valves and causes brake failure; a failed air dryer can also let oil and moisture through.
- Out-of-service leak rate
- A pressure drop greater than 3 psi per minute with brakes released, or greater than 4 psi per minute with brakes applied, is an out-of-service condition.
- Excessive pushrod stroke
- A pushrod stroke beyond the adjustment limit means the slack adjuster needs adjustment or repair; the vehicle is out of service until corrected.
- Air dryer service interval
- The dryer should be serviced at the manufacturer's interval; a saturated desiccant cartridge lets moisture into the system and causes freeze-ups in cold weather.
- Relay valve function
- A valve that speeds up rear brake application by using reservoir air locally rather than waiting for pressure to travel from the foot valve.
- Quick release valve
- A valve that exhausts air quickly when the brake is released, reducing release lag at the wheels and helping prevent dragging brakes.
- ABS braking rule
- Do not pump the brakes on a vehicle with anti-lock brakes; the system modulates pressure automatically to prevent wheel lockup during hard braking.
- Brake drag hazard
- A brake that does not release overheats, can cause a fire, and signals a faulty quick release valve, relay valve, or chamber that must be repaired.