Air Brakes and Braking Systems Flashcards

A 310T technician is diagnosing a tractor-trailer where the spring brakes partially apply during heavy deceleration even though the parking brake is released and system pressure is at 120 psi. Which condition is MOST likely causing this symptom?

Answer: A worn relay valve that cannot maintain piggyback chamber pressure under dynamic load

Under heavy deceleration, the relay valve supplying the spring brake (piggyback) chambers must maintain hold-off pressure against increased demand. A worn relay valve with internal leakage can experience a pressure drop in the spring chamber during dynamic conditions, causing partial spring brake application even though static system pressure appears normal. The spring brake modulator valve is not a standard component sensitive to G-forces, slack adjuster geometry does not interface with the power screw, and a cracked service diaphragm would cause air loss detectable by other means.

When performing a brake balance test on a three-axle transit coach using an electronic brake force meter, the technician finds the rear tag axle produces 18% less retardation force than the drive axle at the same applied pressure. After confirming lining thickness and drum diameter are within spec on all axles, what is the MOST likely root cause?

Answer: Unequal brake chamber sizes between the drive and tag axles creating a force imbalance

When lining and drum specs are confirmed equal, force imbalance at identical line pressure points directly to a mechanical output difference — most commonly unequal brake chamber sizes. A larger chamber produces greater actuator force at the same pressure. Transit coaches often use different chamber types across axles, and a type 24 versus type 30 chamber would produce this measurable retardation difference. A limiting valve restriction would be detectable with a test gauge; contaminated lining was ruled out by the spec confirmation; and a proportioning valve would affect both axles in the rear group equally.

A technician replaces a dual-circuit foot valve on a Class 8 tractor. After installation, a full-pressure brake application is held for 2 minutes with no leakage, but during a slow, light application the secondary circuit pressure lags the primary circuit by 8–10 psi. What is the MOST probable cause of this pressure differential during light applications?

Answer: The graduating spring in the secondary piston of the foot valve has incorrect preload

The dual-circuit foot valve uses stacked pistons with individual graduating springs that control the proportional delivery of pressure to each circuit. If the secondary piston's graduating spring has incorrect preload (due to a faulty replacement part or improper reassembly), it will require more mechanical input force before it begins to open at low pedal effort, causing the secondary circuit to lag during light applications. At full pressure, both circuits eventually equalize, which is why the 2-minute hold test passes. A blockage would cause a flow-dependent lag that worsens at higher flow rates, a sticking check valve is not part of standard dual-circuit valve design, and body deflection from a fitting would be a gross structural issue.

During a pre-trip inspection procedure check on a highway coach, a technician notices the air dryer purges immediately and repeatedly every 15–20 seconds even with the engine running at high idle and no air-consuming accessories active. The compressor cut-out pressure is verified at 120 psi. What does this symptom pattern MOST specifically indicate?

Answer: A high-flow leak in the wet tank is causing the governor to cycle the compressor faster than the dryer's timed purge interval

The pattern — rapid, repeated purges at 15–20 second intervals — indicates the governor is cycling rapidly because system pressure drops quickly after cut-out, forcing the compressor back on and triggering subsequent purges. A high-flow leak in the wet tank (supply side) would cause exactly this: the compressor reaches cut-out, the dryer purges, pressure drops quickly due to the leak, the compressor reloads, builds back to cut-out, and the cycle repeats. A saturated desiccant would affect moisture removal but not purge timing. An unloader valve failure would cause compressor mechanical damage symptoms, not rapid purging. A failed purge solenoid would produce purges at a fixed timer interval regardless of system pressure, not governor-linked intervals.

A 310T technician is evaluating a tractor equipped with an electronically controlled ABS/ATC system. During a loaded brake performance test, the technician observes that the ABS activates on the steer axle at 40 km/h on dry pavement during a moderate brake application that should not exceed 60% of available friction. The wheel speed sensors, tone rings, and wiring check out within spec. What advanced diagnostic step should the technician perform NEXT?

Answer: Measure dynamic wheel speed sensor output voltage under vehicle motion to check for tone ring runout-induced signal dropout

Even when a tone ring and sensor pass static ohm and resistance tests, dynamic testing can reveal runout or damage that causes the sensor output to drop below the ECU's signal threshold momentarily at specific wheel speeds. This creates a false wheel deceleration signal (the ECU interprets signal loss as wheel lock-up) and triggers ABS inappropriately. This is a known failure mode on steered axles due to vibration and steering inputs that affect sensor air gap dynamically. Replacing the ECU without dynamic sensor testing would be a costly misdiagnosis. Slack adjuster over-adjustment would cause brake drag or grabbiness, not ABS activation. Asymmetric application would be caught by comparing chamber strokes but would not explain ABS activation at moderate braking.

A 310T candidate is asked to explain the function of the inversion valve in a tractor protection system. Which statement MOST accurately describes the inversion valve's operation during a trailer supply line failure?

Answer: It uses the loss of trailer supply line pressure as a signal to release a pilot port, allowing spring brake hold-off pressure to exhaust and setting the tractor spring brakes

The inversion valve (also called the spring brake control valve in some configurations) operates on an inverse logic principle: it uses supply pressure at its signal port to hold the spring brake release line open. When the trailer supply line fails or pressure drops below approximately 45 psi, the signal pressure at the inversion valve's pilot port drops, causing the valve to shift and exhaust the spring brake hold-off air, setting the tractor spring brakes automatically. This is 'inverted' because the valve acts when pressure is lost, not when it is applied. It does not proportionally apply spring brakes, does not prevent back-flow (that is the trailer supply valve's function), and does not divert primary circuit air to the trailer.