Air Brake Systems Diagnosis Flashcards

A tractor-trailer exhibits a slow pressure buildup after a full brake application. The governor cuts in and out normally, the air dryer purges correctly, and there are no audible leaks. However, the technician notices the compressor discharge line is unusually hot to the touch. What is the MOST likely cause?

Answer: Worn compressor piston rings allowing excessive blow-by and reduced volumetric efficiency

Worn compressor piston rings reduce volumetric efficiency — the compressor must work harder and run longer to build pressure, generating excessive heat in the discharge line. The governor cycling normally rules out governor faults, and no audible leaks or restriction symptoms point away from the other options. Blow-by from worn rings is a classic cause of both slow pressure buildup and an abnormally hot discharge line.

During a brake adjustment inspection on a straight truck, a technician finds the pushrod stroke on one S-cam brake chamber is within spec at 1.5 inches, but the brake drags after release and the chamber is warm. The slack adjuster and S-cam move freely by hand with the brakes released. What is the MOST likely cause of the dragging brake?

Answer: A weak or broken brake return spring failing to fully retract the shoes

If the slack adjuster and S-cam move freely by hand but the brake drags after air release, the mechanical return path is not fully retracting the shoes — pointing directly to a weak or broken brake return spring. The stroke is within spec, so the adjuster is not the culprit. A ruptured diaphragm would cause air loss, and a cracked drum would more likely cause noise and vibration rather than heat from dragging.

A coach bus with a full-stroke spring brake system fails the 90-second static leakage test — the primary circuit drops 8 psi in 90 seconds with the brakes released. The secondary circuit holds steady. After isolating the primary circuit at the treadle valve, the leak rate drops to zero. What does this indicate?

Answer: The treadle valve primary piston seal is bypassing air to exhaust when released

Isolating the primary circuit at the treadle valve and seeing the leak stop confirms the leak source is in the treadle valve itself — specifically, a worn primary piston seal that allows air to bypass to exhaust in the released position. If the leak were downstream (chamber diaphragm), isolating at the treadle valve inlet would have no effect on the leak rate. The secondary circuit holding steady eliminates cross-port leakage, and a check valve leak would show up in the wet tank pressure, not the primary reservoir.

A technician is diagnosing an ABS fault on a trailer where wheel speed sensor signal is intermittent on one axle end. The resistance across the sensor measures 1,050 ohms — within spec. The air gap is correct. The fault only occurs when the vehicle is moving over rough road surfaces. What is the MOST likely cause?

Answer: Excessive bearing end-play allowing the exciter ring to move laterally and vary the air gap dynamically

Excessive bearing end-play is the classic cause of a vibration-induced intermittent wheel speed sensor fault where static measurements (resistance, static air gap) all pass. As the bearing moves laterally on rough surfaces, the exciter ring shifts in and out of optimal range, causing signal dropouts. The sensor resistance and static air gap are fine, ruling out sensor damage or exciter ring tooth damage. Connector corrosion typically causes a measurable resistance increase rather than only vibration-induced drops.

A heavy truck's front axle service brakes apply fully when the trailer brake hand valve is actuated, but the trailer brakes do not respond. The trailer's supply line shows 120 psi. Which component failure MOST specifically explains this symptom pattern?

Answer: A blocked or closed trailer service (control) line preventing signal pressure from reaching the trailer relay valve

The trailer supply line is pressurized at 120 psi, confirming the glad hand connection and supply path are intact. The hand valve actuating the front tractor brakes confirms the hand valve is sending signal pressure — but only to the tractor circuit, not the trailer. The trailer service (control) line carries the application signal from the hand valve to the trailer relay valve. A blocked or disconnected control line means the trailer relay valve never receives the signal to apply, even though supply air is present. A seized relay valve would prevent application but would not explain why tractor brakes fire.

After rebuilding the foot valve (treadle valve) on a truck, a technician notices that at partial brake applications, there is a notable delay before the brakes release after the pedal is lifted, but full applications release promptly. What assembly error MOST likely caused this?

Answer: The primary and secondary graduating springs were installed in reversed order, altering the graduating force curve

Reversing the primary and secondary graduating springs changes the force-versus-travel relationship of the valve. At low pedal force (partial application), the altered spring stack creates a condition where the graduating piston does not return to the exhaust position promptly — because the spring load balance that normally overcomes delivery pressure at low application levels is disrupted. At full application, line pressure itself overcomes the mechanical resistance and releases normally. An exhaust orifice restriction or blocked port would uniformly slow all releases, not specifically partial ones.