Air Brakes and Braking Systems Flashcards

A truck's brake balance test reveals that the rear axle brakes are applying 200 ms before the front axle brakes during a hard stop. Which condition is MOST likely responsible for this timing discrepancy?

Answer: A faulty front relay valve with excessive delivery volume causing delayed pressure rise

A relay valve with excessive delivery volume (worn or damaged) takes longer to reach cracking pressure and build delivery pressure, causing a delayed application on the axle it serves. The front relay valve would delay front brake application, making the rear brakes apply first. Restricted lines cause slow pressure build but also delay — however, restriction would typically appear as a slow rise rather than a consistent timing offset. Slack adjuster travel and camshaft bushings cause mechanical inefficiency but not the millisecond-precision pneumatic timing offset seen in brake balance tests.

During an inspection, a technician measures 90 psi at the brake chamber when the foot valve is fully applied, but the governor cuts in at 125 psi and cuts out at 145 psi. The brake chambers are 30 type. What is the MOST accurate conclusion?

Answer: The foot valve's graduating spring is weak, limiting maximum delivery pressure

With system pressure at 125–145 psi, a full foot valve application should deliver approximately 100–120 psi to the service chambers. Measuring only 90 psi at full pedal travel with a healthy system indicates the foot valve's graduating spring is not developing full output — a weak or fatigued graduating spring limits the maximum delivery pressure regardless of reservoir pressure. This is a distinct failure mode from a restricted line (which would show as slow build, not a reduced ceiling). The double-check valve and tractor protection valve are not in the service application circuit for a solo tractor.

A technician is diagnosing a vehicle where the spring brakes release normally but automatically re-apply within 45 seconds of release, even with the yellow knob pulled out. Reservoir pressure is stable at 120 psi. What is the MOST likely cause?

Answer: An internal leak in the spring brake chamber piggyback diaphragm is venting supply air to atmosphere

When a piggyback spring brake diaphragm (the service side or the spring brake side) develops an internal leak, supply air to the spring brake chamber bleeds through the leak path into the service port and then to atmosphere through the service brake exhaust. This causes the spring brake supply pressure to decay even though reservoir pressure remains stable — because the loss is localized to that chamber's supply path. The symptom of gradual re-application with stable reservoir pressure and no reservoir leak directly implicates a leaking chamber diaphragm. A park relay valve check valve leak would affect all rear spring brakes simultaneously and be more rapid. The modulating valve is not in the release-hold circuit during normal parking.

On a tractor-trailer combination, the trailer's emergency line loses pressure rapidly but the service line remains pressurized. The red glad hand is secure. Where is the fault MOST likely located?

Answer: An internal failure of the trailer's relay emergency valve causing it to exhaust the emergency port

The trailer relay emergency valve (also called the trailer control valve or emergency relay valve) contains both emergency and service functions. An internal failure — specifically a cracked or displaced piston or failed seat — can cause the valve to exhaust the emergency supply port to atmosphere while still allowing service pressure to operate normally through a separate circuit path. Since the tractor protection valve monitors the emergency line pressure to decide whether to close, a rapid trailer-side exhaust can trigger tractor protection but the fault originates at the trailer valve. A failed tractor protection valve would cut BOTH emergency and service to the trailer. A ruptured reservoir would typically be gradual and audible at the trailer.

A vehicle equipped with ABS experiences wheel lockup on the steer axle during a hard stop on a split-mu surface (ice on left, dry pavement on right). The ABS warning lamp does NOT illuminate. What is the MOST probable explanation?

Answer: The wheel speed sensor air gap on the locking side exceeds specification, generating a false speed signal

If the wheel speed sensor air gap is out of specification (too large), the sensor generates a weaker signal that may read as a speed higher than actual — the ABS ECU sees the wheel as still rolling and does not modulate. This results in lockup without triggering an ABS fault code because the ECU has no indication of a sensor fault; it believes the wheel is rotating normally. A failed-open modulator valve would typically set a fault code (solenoid circuit monitoring). Select-low strategy is valid for some ABS configurations but would prevent lockup, not cause it. ABS ECUs do not have an 'authority limit' that allows lockup without fault.

A technician performs a pushrod stroke measurement on a type 30/30 combination spring brake chamber during a full brake application. The pushrod measures 2.25 inches of stroke. The chamber has a stamped maximum stroke of 2.5 inches. What action is REQUIRED?

Answer: Immediate adjustment is required because the stroke exceeds the 2-inch adjustment limit for a type 30 chamber

The stamped maximum stroke on the chamber is a physical limit, NOT the adjustment standard. Under FMCSA/CVSA brake adjustment criteria (and Canadian equivalents for 310T), a type 30 chamber must be adjusted when stroke equals or exceeds 2 inches (the 'readjustment limit'), regardless of the stamped maximum. At 2.25 inches, the chamber is out of adjustment and requires immediate correction. Waiting to re-test or accepting any stroke below the stamped maximum are common errors — the adjustment standard is stricter than the physical limit. Combination spring brake chambers use the same stroke limits as standard service chambers of the same size.