Air Brake Systems Diagnosis Flashcards

A vehicle's air dryer purge valve is cycling every 30 seconds during a stationary governor cut-in/cut-out test. The compressor unloads normally at 125 psi and cuts in at 100 psi. What is the MOST likely cause of the rapid purge cycling?

Answer: A leaking delivery check valve inside the air dryer allowing reverse flow through the desiccant cartridge

A leaking delivery check valve inside the air dryer allows downstream air to bleed back through the desiccant bed, triggering the purge timer repeatedly even without compressor cycling. The compressor itself is functioning normally (correct cut-in/cut-out pressures), ruling out governor issues. A saturated cartridge affects drying efficiency but does not cause mechanical purge cycling. Tank sizing affects cycle frequency but not the purge valve itself.

During a brake adjustment inspection on a tractor with S-cam drum brakes, you find that the slack adjuster on the left-front steer axle moves 2.5 inches at the clevis pin hole before any resistance is felt, but the push rod stroke at full application is only 1.6 inches. What does this combination MOST likely indicate?

Answer: The automatic slack adjuster is slipping on the camshaft splines, creating false free-play measurement

High free-play at the clevis combined with a short actual push rod stroke is the classic signature of an ASA slipping on camshaft splines. The adjuster rotates freely instead of advancing the cam, so the clevis feels loose but the stroke never exceeds the cam-over threshold. Worn linings would produce excessive stroke, not reduced stroke. An undersized chamber would show insufficient stroke but not the large free-play at the clevis. Foundation brake adjustment is clearly abnormal here.

A technician performs a spring brake functional test and finds that when the spring brake control valve is moved to the 'park' position, the rear wheels lock immediately, but when the valve is moved back to 'release', one piggyback spring brake chamber on the right-rear axle takes 45 seconds to fully release. Supply pressure is 120 psi. What is the MOST probable diagnosis?

Answer: A collapsed or kinked air line between the relay valve and the slow-releasing chamber

A collapsed or kinked line between the relay valve and the slow chamber creates a restriction that severely limits air flow rate into the piggyback chamber. At rest, full pressure can eventually build across a restriction, so the brake does finally release, but it takes far longer than normal. A faulty modulator would affect all chambers on that circuit. A double-check valve fault typically causes complete non-release or pressure imbalance across axles. Spring brake chambers have no return spring — the spring itself provides clamping force and air provides release force against it.

A coach equipped with an ABS/ATC system exhibits the following symptom: the ATC light illuminates and the system reduces engine torque momentarily at speeds above 55 mph on dry pavement on straight roads, with no wheel slip present. Wheel speed sensor outputs are all within 2 rpm of each other. What should the technician suspect FIRST?

Answer: A cracked or missing tone ring tooth creating a false drop-out signal at higher rotational speeds

A cracked or missing tone ring tooth produces a momentary dropout in wheel speed signal. At low speeds this dropout may be too brief to trigger ATC logic, but at higher speeds the wheel rotates faster, making the dropout longer in duration (more time between pulses) relative to the adjacent teeth signals — crossing the ECU's slip detection threshold. All four sensors reading within 2 rpm is a static measurement; the fault is dynamic and speed-dependent. A corrupted ECU would typically set multiple fault codes and affect lower speeds too. ATC modulator contamination and TPS issues would present very differently.

When testing a tractor protection valve, a technician disconnects the gladhands and observes that the tractor supply line (emergency line) drops from 120 psi to 0 psi within 8 seconds with the engine running and governor maintaining system pressure. What condition does this MOST accurately indicate?

Answer: The tractor protection valve has a damaged seat, preventing it from closing when line pressure drops below the trip threshold

The tractor protection valve should CLOSE the emergency supply line when line pressure drops below approximately 20–45 psi (the trip threshold), isolating the tractor system from an open gladhand. A rapid drop to 0 psi with a running compressor means the valve is not closing — a damaged seat or broken internal spring prevents sealing, allowing the compressor to continuously vent through the open line. If it were functioning correctly, the valve would close and the tractor tanks would hold pressure. A stuck-open valve description is close, but the root cause is seat damage preventing closure at the trip threshold, not a valve mechanically jammed open at all times.

A vehicle with full air brakes and an inversion valve (often called a 'ratio valve' or 'relay emergency valve') on the front axle shows excessive front brake force during a light stop — front wheels lock before rears during a moderate-pressure pedal application. After confirming correct lining and drum condition on all axles, what is the MOST likely cause?

Answer: The inversion valve is operating in reverse: applying high pressure to front brakes during low-pedal inputs due to a broken spring or damaged piston

An inversion valve (front axle ratio valve) is designed to deliver REDUCED pressure to front brakes relative to the delivery signal during light applications, protecting steer axle stability. If the internal spring or piston is damaged, the valve can invert its function — delivering proportionally HIGHER pressure to fronts during light pedal input, causing front lockup. A limiting valve stuck in highway mode would reduce front pressure, not increase it. Glazed linings can increase static friction but the symptom would be unpredictable, not systematic. A rear relay valve lag would cause rear underbraking — a separate problem — but would not directly cause front lockup.