Pennsylvania CDL Air Brakes flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Air compressor

Engine-driven pump that builds and maintains air pressure in the brake system; the governor controls when it loads and unloads, typically cutting out near 120-125 psi and cutting in near 100 psi.

Governor

Valve that senses system pressure and controls the compressor, unloading it at cut-out pressure near 120-125 psi and loading it again at cut-in pressure near 100 psi.

Air dryer

Device after the compressor that removes moisture and oil vapor from the air; a failed dryer lets water reach tanks and valves, where it can freeze and cause brake failure.

Supply (wet) tank

First tank after the compressor and dryer; it collects condensed water and feeds the other tanks through one-way check valves, and it carries the safety valve.

One-way check valve

Valve between tanks that allows air to flow in only one direction, keeping a leak in one part of the system from draining the whole air supply.

Foot valve (brake valve)

Treadle-operated valve that meters air from the tanks to the brake chambers in proportion to pedal travel, giving the driver control of braking force.

Relay valve

Valve that speeds brake application and release by sending tank air directly to the brake chambers instead of routing all the air through the foot valve.

Slack adjuster

Link between the brake chamber pushrod and the brake shoes; with brakes released, pushrod free play should be no more than about 1 inch on a standard chamber.

Safety valve

Valve on the supply tank that releases air if pressure rises above its setting, typically around 150 psi, protecting the system from overpressure.

Dual air brake system

Two separate circuits, usually primary for rear brakes and secondary for front brakes, each with its own tank and check valve, sharing one compressor and supply tank.

Circuit isolation

One-way check valves separate the two circuits so a failure in one does not drain the other; the remaining circuit still provides braking.

Dual foot valve

Brake valve with two separate supply and delivery sections, one per circuit, so an internal seal failure affects only one circuit of the dual system.

Tractor protection valve

Valve on combination vehicles that closes and prevents air loss from the tractor if the trailer breaks away or a trailer air line is severed.

Buildup time

With the engine at fast idle and the system fully charged, pressure should build from 85 to 100 psi within 45 seconds; longer time indicates a compressor or leak problem.

Static leak test

With the engine off and system charged, hold the foot brake one minute; loss should not exceed 3 psi for a single vehicle or 4 psi for a combination.

Low pressure warning test

With the engine off, pump the brake and confirm the light and buzzer activate before pressure in either circuit falls below 60 psi.

Spring brake test

With the engine off and pressure below 60 psi, continue pumping until the parking brake valve pops out between roughly 20 and 45 psi, confirming automatic application.

Parking brake hold test

Apply the parking brake with the transmission in a low gear and attempt to pull forward; the vehicle must not move, confirming the spring brakes hold.

Tank draining

Draining moisture from the supply and service tanks at the end of each day of operation prevents water from reaching valves and freezing in cold weather.

Applied leak test

With the engine off and the brakes held, pressure loss should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

Supply-side leak

A steady hiss with the brakes released, coming from a tank, hose, or fitting, indicates a leak on the charged side of the system that must be repaired.

Delivery-side leak

A hiss heard only when the brakes are applied points to a leak in a brake chamber, hose, or valve on the delivery side of the system.

Continuous compressor running

A compressor that never unloads, or a governor that never cuts out, usually indicates an air leak or a faulty governor rather than normal high demand.

Low air pressure warning

Visible and audible signal, typically a red light and buzzer, that must activate before pressure in either circuit falls below 60 psi.

Warning device failure

A warning that fails to activate, or activates only at very low pressure, is a defect that places the vehicle out of service until repaired.

Warning activation point

The low pressure warning must come on well before the spring brakes apply automatically at roughly 20 to 45 psi, giving the driver time to stop safely.

Spring brakes

Brakes held applied by a heavy spring and released by air pressure; when system pressure is lost, the spring applies the brakes automatically.

Spring brake application point

Spring brakes apply automatically when system pressure drops to roughly 20 to 45 psi, which is why the low pressure warning must activate before 60 psi.

Parking brake valve

Yellow diamond knob that releases air from the spring brake chambers to apply the brakes; pushing it in recharges the chambers and releases the brakes.

Caging a spring brake

Mechanically compressing and locking the spring to release the brake for moving a disabled vehicle only; caged brakes provide no emergency braking and must not be driven normally.

Parking on a grade

Set the parking brake, place the transmission in the lowest gear or park, and use wheel chocks where required; confirm system pressure is normal before releasing the brake.

Air brake following distance

Air brakes stop more slowly than hydraulic brakes; allow at least one second per 10 feet of vehicle length plus one second, and more in poor conditions.