Five free practice questions on air brakes and air systems: block C of the Red Seal 310T Truck and Coach Technician exam, about 17 of its 135 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.
01The questions
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A tractor's air pressure builds normally, but as soon as the driver applies the brakes with the engine off, air can be heard escaping at the treadle valve and the pressure falls quickly. With the brakes released the system holds. What does this indicate?
- A leaking gladhand seal at the trailer.
- A leaking treadle valve or diaphragm.
- A faulty governor.
- A saturated air dryer.
Show the answer
B. Correct. A leak that appears only with the brakes applied is on the delivery side of the service system: the treadle valve's internal seals leaking to exhaust when applied, or a service chamber diaphragm passing air, which would show at the chamber's vent. The RSOS lists air leaks among symptoms, valves and brake chambers among components, and requires the technician to interpret the test result to determine the next step. Soapy water at the exhaust port and the chambers locates it; the valve or chamber is replaced. NSC 11 rejects a detectable leak at any location.
Why not the others
A. A gladhand leak is present whenever the trailer supply is charged, with or without a brake application.
C. The governor controls compressor loading; it does not leak when the brakes are applied.
D. A dryer problem shows as moisture in the tanks or a continuous purge, not an application-dependent leak.
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A tractor's ABS warning lamp stays on. The scan tool shows a fault for the left rear wheel-speed sensor with an erratic signal, and the sensor's circuit resistance and connector test good. What should the technician check?
- Sensor gap, tone ring and bearing play.
- The air dryer.
- The brake linings on that wheel.
- The ABS module, which should be replaced.
Show the answer
A. Correct. An erratic (not absent) wheel-speed signal with a sound circuit is a mechanical signal problem: the sensor has backed away from the tone ring (it is pushed in to touch the ring, which sets the gap), the ring has damaged, dirty or missing teeth, or excessive wheel-bearing end play lets the ring wobble. The RSOS lists ABS lights on among symptoms, sensors, wiring, ECMs and modulating valves among ABS components, and the ABS scan tool and digital voltmeter among tools; ABS sensors are among components adjusted. The sensor is reseated, the ring inspected and bearing play measured.
Why not the others
B. The dryer is part of the air supply and has no role in the ABS sensor signal.
C. Lining condition does not affect the wheel-speed signal.
D. The module correctly reports an erratic signal from one wheel; that is the module working. The fault is at the wheel end.
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A technician replaced a tractor's air compressor because it was pumping oil. Two weeks later the truck is back with oil in the tanks again. What was most likely missed?
- The governor cut-out was set too low.
- A restricted intake and a saturated dryer.
- The wrong engine oil grade is in the engine.
- The new compressor is defective.
Show the answer
B. Correct. A restricted intake pulls a vacuum in the cylinder that draws oil past the rings, so a new compressor on the same restricted intake pumps oil just as the old one did. A saturated dryer also passes the oil aerosol it should trap, and oil left in the tanks and lines after the swap keeps showing up in the drains. The RSOS requires repairs to replace the parts causing the failure, not just the symptom; NSC 11 rejects a compressor air filter contaminated enough to restrict air flow. The intake, dryer and tanks are addressed with the compressor.
Why not the others
A. Cut-out pressure does not cause a compressor to pass oil.
C. Engine oil grade does not cause oil pumping when the compressor's intake and dryer are the actual restriction.
D. A second failure with the same symptom points to a common cause that was not corrected, not to two bad compressors.
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According to NSC Standard 11 and CMVSS 121, which vehicles must be equipped with brake pushrod stroke indicators, and what must the indicator show?
- From May 31, 1996; must show over-stroke.
- Only vehicles with manual slack adjusters.
- No vehicles; indicators are optional.
- Only trailers built after 2010.
Show the answer
A. Correct. NSC 11 notes that brake pushrod stroke indicators are required by CMVSS 121 on vehicles manufactured on and after May 31, 1996; they normally consist of markings on the pushrod but can be mounted on or adjacent to the linkage, and they must be capable of showing an over-stroke condition. A missing required stroke indicator is a reject condition. The indicator lets an inspector or driver spot an over-stroked brake at a walk-around, but the technician still measures actual stroke at the PM as the standard requires.
Why not the others
B. The requirement is by manufacture date, regardless of adjuster type; automatic adjusters can over-stroke too.
C. They are mandated by CMVSS 121 for the stated vehicles, and NSC 11 rejects a missing required indicator.
D. The requirement applies to air-braked vehicles generally from May 31, 1996, not only to newer trailers.
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On a tractor's dual-circuit air brake system, what is the function of the one-way check valves at the primary and secondary tanks, and how does NSC Standard 11 require them to be tested?
- They isolate each tank; drain the wet tank, no drop.
- They regulate tank pressure to the governor's cut-out value.
- They drain moisture automatically.
- They are not tested; they are replaced at every PM.
Show the answer
A. Correct. The one-way check valves between the supply (wet) tank and the primary and secondary tanks let air in but not out, so a leak in one service circuit cannot bleed the other tank down. NSC 11 (Section 3A, item 5) tests them by opening the supply tank drain at normal operating pressure and rejects the vehicle if pressure drops in either the primary or secondary tank; an integral-dryer system with no wet tank is tested per the manufacturer, and an inoperative or missing check valve is a hazardous condition. The RSOS lists valves and tanks among air-system components.
Why not the others
B. Pressure regulation is the governor's job. Check valves only control direction of flow.
C. Moisture is handled by the dryer and the tank drain valves or automatic ejectors, not the check valves.
D. Check valves are tested per NSC 11 and replaced only if inoperative.
02Answer key
| Question | Answer | Sub-task |
|---|---|---|
| Q1 | B | Diagnoses air systems |
| Q2 | A | Diagnoses brake systems |
| Q3 | B | Repairs air systems |
| Q4 | A | Services brake systems |
| Q5 | A | Services air systems |
03What block C covers
This block covers air supply and brakes on trucks, tractors and coaches. It includes compressors, governors, air dryers and tanks, check valves, treadle and relay valves, low air warnings, spring brakes and caging, brake chambers, slack adjusters and pushrod stroke, drums, linings and air disc brakes, ABS wheel-speed sensors, and hydraulic brakes, all checked against NSC Standard 11.
The Red Seal Occupational Standard names this block “Services, diagnoses and repairs air systems and brake systems”. It carries 12.6% of the exam, about 17 of the 135 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.
| Task and its sub-tasks | Share of block |
|---|---|
| Services, diagnoses and repairs air systems Services air systems; Diagnoses air systems; Repairs air systems | 51% |
| Services, diagnoses and repairs brake systems Services brake systems; Diagnoses brake systems; Repairs brake systems | 49% |
04What the questions turn on
Measuring pushrod stroke
Stroke is measured with 620 to 690 kPa (90 to 100 psi) in the tanks, engine off, service brakes fully applied. A Type 30 standard-stroke clamp chamber is rejected past 51 mm, as is an axle whose two strokes differ by over 6 mm.
Auto slacks need repair, not adjustment
An automatic slack adjuster at or past the stroke limit means something is stopping it from working, such as worn cam bushings or a seized adjuster. A manual adjustment will not correct it, and it cannot be swapped for a manual adjuster.
Air system test numbers
Under NSC 11, the low air warning must come on as pressure falls below 414 kPa (60 psi), governor cut-out may never exceed 1000 kPa (145 psi), and leakage over 7 kPa (1 psi) per minute is rejected.
Water and oil in the tanks
Water and milky oil from the supply tank mean the air dryer is not removing moisture. If a compressor pumps oil, check for a restricted intake and a saturated dryer too, or the new compressor will pump oil the same way.
05The other blocks
The 310T exam has 11 blocks. Each has its own page of free questions:
The 310T Truck and Coach Technician practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.