Saskatchewan Air Brake: Air Brake Components

Air Brake Components

This domain tests component function and how a fault shows up. The exam may describe a symptom and ask which valve, chamber or adjustment is involved, so learn the air path and the purpose of each part.

Core rules

Compressor: engine-driven air source. Governor: controls loading/unloading by cut-in and cut-out pressure. Air dryer: removes moisture and oil; a failed dryer sends contamination downstream.
Reservoirs: wet/supply, primary and secondary. Check valves and pressure-protection valves isolate circuits and preserve emergency pressure.
Foot valve: driver's control signal. Relay valve: supplies reservoir air to rear chambers when it senses the control signal. Quick-release valve: speeds release by exhausting chamber air locally.
Brake chamber: converts air pressure to pushrod force. Slack adjuster: converts pushrod travel into S-cam rotation and multiplies leverage. S-cam, wedge or disc mechanism: applies lining to drum or rotor.
Spring brake chamber: contains a strong spring for parking/emergency. Air pressure compresses the spring to release the brake. Inversion or relay valves may control spring-brake application on some systems.

Worked example

A driver reports that rear brakes apply normally but release slowly, with a hiss near the rear axle after releasing the pedal. Reasoning: the control signal and relay valve are likely working because application occurs; slow release points to restricted exhaust or a faulty quick-release/relay exhaust. Check for crushed hose, ice, dirt or a failed valve. A chamber problem would more likely affect one side.

Traps

Do not call the foot valve a relay valve; the foot valve controls, the relay valve supplies volume. Do not confuse the air dryer with a reservoir. Do not say the governor builds pressure-it controls when the compressor builds. Do not treat a spring-brake chamber as a service chamber. Do not diagnose by replacing parts without checking air flow and exhaust.

Maintenance and Adjustments

This domain tests preventive maintenance, legal adjustment limits and safe repair practices. The exam expects you to know when a brake is out of adjustment and when a component must be replaced rather than “adjusted around.”

Core rules

Follow the manufacturer's maintenance intervals and daily inspection routine. Repair air leaks; do not rely on the compressor to overcome them.
Service the air dryer per specification. Oil or water in reservoirs indicates contamination that can damage valves and chambers.
Check pushrod stroke at full pressure with the brake applied, using the chamber's stroke limit chart. Measure from a marked pushrod or reference point. Excessive stroke means the foundation brake is out of adjustment or worn.
Automatic slack adjusters still require stroke checks. If an auto slack is out of adjustment, investigate pushrod geometry, chamber size, lining wear and installation; do not repeatedly manual-adjust it to hide a fault.
Replace contaminated linings, cracked drums, damaged hoses and leaking chambers. Never disassemble a spring-brake chamber; use the caging bolt only for safe removal per procedure.

Worked example

A mechanic measures 2¼ inches of pushrod stroke on a chamber whose chart limit is 2 inches. The lining looks thick. Reasoning: the brake is over the legal stroke limit regardless of lining appearance. Check the slack adjuster, pushrod, chamber and S-cam bushings. Adjust only per the manufacturer's procedure, then re-measure. If the auto slack will not hold adjustment, repair the cause.

Traps

Do not adjust a slack adjuster without checking stroke and foundation condition. Do not use lining thickness alone to pass a brake. Do not back off an adjustment to make a stroke gauge fit. Do not open a spring-brake chamber. Do not ignore an air dryer that is past its service life.