Ontario Air Brake Z Test flashcards

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

Governor cut-out pressure

The reservoir pressure at which the governor signals the compressor to stop pumping, typically about 830 kPa (120 psi) in a heavy vehicle air brake system.

Governor cut-in pressure

The pressure at which the governor lets the compressor resume pumping, typically about 690 kPa (100 psi), restoring the reservoir supply after a drop.

Air dryer function

Removes moisture and oil from compressed air before it reaches the reservoirs, using desiccant and a purge cycle that expels collected water at each unload.

Wet tank purpose

The first reservoir to receive compressor output; it condenses and collects most moisture and oil, and is drained daily to protect downstream components.

Relay valve role

A valve that uses a small control signal to supply a large volume of air from a nearby reservoir, speeding brake application and release on long wheelbases.

Spring brake operation

Parking brakes held applied by spring force and released by air pressure; loss of air pressure causes the springs to apply the brakes automatically.

Dual-circuit system

A split air brake design separating front and rear service circuits so one failure leaves the other circuit with stored air and triggers a warning.

Quick-release valve

A valve mounted near the wheel that exhausts service-chamber air locally, speeding brake release and reducing the time air must travel back to the foot valve.

Static leak test

With the engine off and full service brake applied for one minute, gauge drop must stay within the vehicle's inspection limit, commonly about 21 kPa (3 psi).

Compressor build-up test

From about 550 kPa (80 psi) at fast idle, pressure should reach cut-out within the specified time; slow build indicates compressor, governor, or leak problems.

Low-pressure warning test

With the key on and engine off, slowly reduce pressure; the buzzer or light must activate before pressure falls below about 380 kPa (55 psi).

Pushrod stroke check

Mark the pushrod, apply full brake, and measure travel; over-stroke beyond the vehicle's limit indicates the slack adjuster or foundation brake needs service.

Spring brake test

With full system pressure, apply and release the parking brake; the spring brakes must apply and release fully with no dragging or delay.

Daily reservoir draining

Open the drain valves on all reservoirs daily to remove water and oil; a wet tank that discharges oil or water signals dryer or compressor trouble.

Pre-trip brake sequence

Drain tanks, check build-up and governor cut-in/cut-out, check warning devices, check spring brakes, then confirm service brake response at low speed.

Applied-pressure leak check

With the engine off and full brake held, a steady pressure drop indicates a service-circuit leak and the vehicle must be taken out of service until repaired.

Slow pressure build causes

Compressor wear, a slipping or broken drive belt, a restricted intake, a leaking discharge line, or a faulty governor or unloader.

Over-pressure cause

Pressure rising above cut-out or venting at the safety valve indicates a stuck governor, a failed unloader, or a blocked signal line to the governor.

Saturated air dryer

A dryer with exhausted desiccant or a stuck purge valve passes moisture into the wet tank; service the cartridge and check the heater in cold weather.

Slow brake release cause

A restricted line, a plugged exhaust port on the relay or quick-release valve, or a kinked hose delays exhausting air and slows brake release.

Dragging brake causes

A stuck relay valve, a seized slack adjuster, a broken return spring, or a restricted exhaust; dragging generates heat that can damage brakes or cause fire.

Circuit isolation diagnosis

Hold the brake and watch which gauge drops to identify the failed circuit, then inspect that circuit's chambers, hoses, and valves.

Automatic slack adjuster rule

Do not manually adjust an auto slack to hide an over-stroked pushrod; over-stroke usually means a worn S-cam, drum, or lining, or a broken adjuster.

Alcohol evaporator limits

An alcohol evaporator provides freeze protection only; it does not remove moisture and must be maintained at the correct level, and it does not replace daily draining.

Air brake stopping distance

Air brakes take longer to apply and release than hydraulic brakes, so a longer following distance is needed to allow for pressure build and lag time.

Downgrade braking technique

Select a low gear and brake in short, firm applications, letting speed recover between them; continuous light braking overheats drums and causes fade.

Do not fan the pedal

Each application uses stored air; repeated fanning can exhaust the supply, trigger the low-pressure warning, and apply the spring brakes.

Low-pressure warning response

When the warning sounds, stop safely as soon as possible; continued driving risks automatic spring brake application at a dangerous location.

Before a long descent

Confirm the pressure gauge is at normal operating range and select a gear that holds the desired speed without needing continuous service braking.

Cold-weather moisture risk

Drain reservoirs before driving and after stops in freezing weather; trapped moisture can freeze in lines and valves and cause brake failure.

Parking on a grade

Apply the parking brake, chock the wheels if on a slope, and confirm the spring brakes are fully set because pressure can bleed off over time.

Test after brake work

After any brake repair, perform a full leak and function test at low speed before normal service; an untested repair is not known to be safe.