Air Brakes and Braking Systems Flashcards

A truck's air brake system exhibits a slow pressure build-up rate after the compressor reaches cut-in pressure. The governor and unloader valves test correctly. What is the MOST likely cause?

Answer: Worn compressor piston rings causing excessive blow-by

Worn compressor piston rings allow combustion gases and air to bypass the rings (blow-by), reducing the compressor's volumetric efficiency. This directly reduces the rate at which the system builds pressure. A stuck purge valve would cause continuous air loss but the symptom would be failure to hold pressure, not slow build-up. A defective relief valve would vent only above a set pressure, not affect build-up rate. A cracked check valve seat would cause back-bleeding between reservoirs but not inherently slow build-up.

During a brake balance test on a tandem-axle tractor, the forward-rear axle shoes show significantly more wear than the rear-rear axle. Which condition BEST explains this imbalance?

Answer: An out-of-specification pushrod stroke on the rear-rear axle chambers

An out-of-specification (excessive) pushrod stroke on the rear-rear axle means those brakes apply later in the pedal travel and with less mechanical advantage, so the forward-rear brakes absorb disproportionately more of the braking load, causing accelerated wear. A leaking quick-release valve on the forward-rear circuit would reduce that axle's braking force, not increase it. A restricted relay valve exhaust on the rear-rear would slow release (dragging), not reduce application force. Spring brake hold-off pressure affects parking, not service brake balance.

A technician performs an ABS functional test and finds that the front axle modulator valve cycles correctly during a controlled stop, but the rear axle modulator never cycles even on a slippery surface. The wheel speed sensors on all axles have correct and equal resistance readings. What is the MOST probable fault?

Answer: Incorrect reluctor ring tooth count on the rear axle installed during a differential service

If a replacement reluctor (tone) ring with an incorrect tooth count was installed, the ECU receives a wheel speed signal that does not match the programmed tooth count, causing it to calculate erroneous wheel speed data. On a slippery surface, the rear wheels may lock, but the ECU does not recognize the deceleration rate as wheel slip based on the miscalculated speed — so it never commands modulation. Sensor resistance checks only confirm wiring integrity, not signal frequency accuracy. A defective ECU output driver would be flagged as a fault code. A restricted shuttle valve would affect braking force uniformly and would be apparent as a brake imbalance, not ABS non-function.

A spring brake actuator (piggyback chamber) is being replaced. After installation and system pressurization, the parking brake releases correctly, but during a full-pressure service brake application the pushrod travel is 25% less than the adjacent chamber on the same axle. The new chamber's clamp band and mounting are correct. What should the technician check FIRST?

Answer: The size rating of the replacement chamber versus the original specification

Piggyback spring brake chambers are available in multiple size combinations (e.g., 30/30, 24/24, 30/24). If the wrong service-side chamber size was installed, its diaphragm area is smaller, producing less pushrod force and travel for a given air pressure. This is a common error during replacement. A manufacturing defect in the diaphragm would more likely cause complete failure or severe air leakage. Hold-off pressure affects only the spring section release, not service brake stroke. Slack adjuster arm length is a separate assembly and would not change the chamber's pushrod travel.

A coach equipped with a dual-circuit air brake system experiences a gradual loss of brake effectiveness over a 45-minute highway run, but the brake system passes all static tests when cold. Reservoir pressures remain normal throughout the run. What is the MOST likely cause?

Answer: Brake drum thermal expansion causing reduced lining-to-drum contact

As cast iron brake drums heat up from repeated braking during a long highway run (especially downhill or in stop-and-go traffic), they expand radially. This increases the effective drum diameter, increasing the lining-to-drum clearance and requiring more pushrod stroke to make contact. If the slack adjusters are at the maximum allowable stroke, this thermal expansion can push the effective stroke past the cam's optimal mechanical advantage range, reducing braking force — a phenomenon called 'brake fade due to drum growth.' Air brakes do not use brake fluid, eliminating vapor lock. The governor regulates compressor cycling regardless of temperature. Relay valve thermal expansion is not a recognized failure mode.

According to CMVSS regulations governing air brake systems in Canada, what is the maximum allowable brake chamber pushrod stroke for a Type 30 long-stroke brake chamber at the point of brake adjustment?

Answer: 2.5 inches (63.5 mm)

For a Type 30 long-stroke brake chamber, the maximum allowable readjustment limit under CMVSS/FMCSS 121 is 2.5 inches (63.5 mm). Standard Type 30 chambers have a limit of 2 inches (50.8 mm), but the long-stroke variant is specifically designed for greater travel to accommodate automatic slack adjusters and is rated to 2.5 inches. Exceeding this stroke means the chamber is operating past its designed mechanical advantage range and the brakes are considered out of adjustment. Knowing the distinction between standard and long-stroke chamber limits is essential for roadside inspections and proper brake adjustment certification.