Five free practice questions on air brakes: content area A of the ASE T4 Brakes test, 32 of its 50 scored questions (64%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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A truck's low-air warning is lit by a pressure switch that closes when service reservoir pressure is low. With a test gauge teed in at a service reservoir and the key on, how is the pressure at which the warning comes on checked?
- Drain the supply reservoir and read the gauge as the warning lights
- Idle the engine and read the gauge as the governor cuts in
- Fan the brakes with the engine off and read the gauge as it lights
- Unplug the warning switch with the system full and watch the warning
Show the answer
C. Correct. With the engine stopped the compressor cannot refill the system, so each application lowers service reservoir pressure a step at a time; the test gauge reading when the warning comes on is compared with the manufacturer's setting. Then restart the engine and confirm the warning goes out as pressure builds.
Why not the others
A. Each service reservoir is protected by a one-way check valve, so draining the supply reservoir leaves the service reservoirs charged and a good warning stays off. Service reservoir pressure is lowered by fanning the brakes with the engine stopped.
B. The governor cuts in above the low-air warning point, so a working warning is off at cut-in and this reading says nothing about the switch. The warning point is found by fanning the brakes down with the engine stopped.
D. With the system full the switch is open and the warning is already off, so unplugging it changes nothing and measures no setting. The warning point is read from the test gauge while the brakes are fanned down with the engine stopped.
ID T4-A-004 · Task T4-A-1.5 (ASE task list)
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On an engine whose power steering pump is driven off the back of the gear-driven air compressor through a splined coupling, what happens if that coupling fails?
- Air pressure stops building while steering stays normal
- Steering turns hard while air pressure builds normally
- Steering turns hard and air pressure stops building
- The compressor keeps pumping and will not unload
Show the answer
B. Correct. The coupling only passes drive from the compressor crankshaft to the pump, so when it fails the pump stops turning while the compressor, still driven by its gear, keeps building air. Inspect the coupling and both sets of splines whenever the compressor or pump is removed.
Why not the others
A. That is what a stripped compressor drive gear would do if the pump were driven some other way. A failed coupling behind the compressor leaves the compressor running and stops only the pump.
C. Both would stop only if the compressor's own drive failed, since the pump is driven through the compressor. A failed coupling between them stops the pump alone.
D. Unloading is controlled by the governor and the unloader, not by the pump coupling. A failed coupling stops the power steering pump and leaves the air system working normally.
ID T4-A-025 · Task T4-A-1.8 (ASE task list)
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One drive-axle S-cam brake with an automatic slack adjuster drags and runs hot after each stop, though a gauge shows no air left in its service chamber and its spring brake is held fully released. Any of these could be the cause EXCEPT:
- a pushrod binding on the chamber housing opening.
- an out-of-round brake drum.
- a worn clutch inside the slack adjuster.
- a control arm set in the wrong position.
Show the answer
C. Correct. A worn clutch slips, so the adjuster loses the adjustment it has taken up; lining clearance grows, and the result is a long pushrod stroke and weak braking, not drag. A brake that drags with no air in its chamber has something holding the shoes against the drum.
Why not the others
A. A pushrod that binds on the chamber housing cannot retract fully, so it holds the slack adjuster and cam partly applied with no air in the chamber. This could cause it.
B. An out-of-round drum comes closer to the linings at its high spots, so the shoes rub there on every turn even with the brake released. This could cause it.
D. The control arm is the adjuster's reference; set in the wrong position at installation, it makes the adjuster keep the shoes too close to the drum, so the brake runs tight and drags. This could cause it.
ID T4-A-045 · Task T4-A-2.1 (ASE task list)
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A tractor stops poorly, and one drive-axle brake's stroke is over its readjustment limit at the specified application pressure. Its automatic slack adjuster passes the manufacturer's function test, turning its adjusting nut on each application. Any of these could be the cause EXCEPT:
- clevis pins and clevis holes worn oversize.
- linings with a higher friction rating.
- camshaft bushings worn oversize.
- a worn bushing in the adjuster's clevis bore.
Show the answer
B. Correct. A lining's friction rating changes how much braking torque the shoes make, not how far the pushrod travels, so it cannot put the stroke over its limit, and higher friction would not make the truck stop poorly. A long stroke with a working adjuster comes from lost motion in the linkage or foundation brake.
Why not the others
A. Worn clevis pins and holes add free motion between the pushrod and the adjuster that an automatic adjuster cannot take up, so the stroke stays long with the adjuster working. This could cause it.
C. Worn camshaft bushings let the camshaft shift sideways under load instead of turning the shoes out, and that lost motion adds to pushrod travel. This could cause it.
D. A worn bushing in the adjuster arm's clevis pin bore adds free motion at the clevis joint that the adjuster cannot remove, so the stroke grows. This could cause it.
ID T4-A-069 · Task T4-A-2.1 (ASE task list)
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What does the SAE J866 edge code printed on a brake lining identify?
- The highest drum temperature the lining tolerates
- The lining's rated wear life in miles
- The lining's friction coefficient classification
- The axle weight rating the lining is approved for
Show the answer
C. Correct. The edge code classifies the lining's friction coefficient from a standardized lining quality test under SAE J866. It is a classification, not a performance guarantee, so replacement linings are chosen to the friction rating the vehicle or axle manufacturer specifies.
Why not the others
A. The code is not a temperature limit. It classifies the lining's friction coefficient as measured in a standard lining test.
B. The code says nothing about wear life, which depends on duty cycle, drum condition and adjustment. It classifies the lining's friction coefficient.
D. The code does not give an axle rating; it classifies friction coefficient, and that rating is matched to what the axle or vehicle manufacturer specifies.
ID T4-A-095 · Task T4-A-2.7 (ASE task list) · SAE J866
02Answer key
| Question | Answer |
|---|---|
| Q1 | C |
| Q2 | B |
| Q3 | C |
| Q4 | B |
| Q5 | C |
03What area A covers
This content area deals with the air brakes on trucks and tractors, in three parts. The supply and service side runs from the compressor, governor and air dryer through the reservoirs, foot valve, relay valves and tractor protection valve to the trailer. The foundation side covers chambers, slack adjusters, S-cams, air disc brakes, drums, rotors and wheel hubs. The parking side covers spring brake chambers, their controls, caging and interlocks.
The ASE test specification calls this content area “Air Brakes Diagnosis and Repair”. It carries 32 of the 50 scored questions, 64% of the test.
- Time air build-up and measure released and applied leakage
- Service compressors, governors, unloaders, air dryers and reservoirs
- Test gauges, pressure sensors and low-air warning circuits
- Test foot, relay, hand control and tractor protection valves
- Tell tractor faults from trailer faults
- Service chambers, slack adjusters, S-cams and air disc calipers
- Inspect linings, drums, rotors, hubs and wheel bearings
- Test, cage and replace spring brakes and parking interlocks
04What the questions turn on
Timing an applied leakage test
Applied leakage is timed from where the gauge settles after the application, since the first drop is air filling chambers and lines. A system that passes released but fails applied is leaking on the delivery side: foot valve, relay valves, lines or service chambers.
Governor or unloader
A gauge teed into the unloader line splits the two. If the governor's signal arrives at cut-out but pressure keeps climbing, the unloader is not responding. If the signal arrives well below cut-in, the governor is at fault and the unloader is just obeying it.
Stroke over the limit
An automatic slack adjuster holds stroke in range by itself, so a brake over its readjustment limit points to a faulty adjuster, worn foundation parts or an installation error. Turning the adjusting nut only hides it. Stroke is measured with the spring brake held released.
Caging a spring brake safely
Caging a spring brake releases that parking brake, so the wheels are chocked first. Hold-off air compresses the power spring, which lets the release bolt nut run down by hand with little effort. The spring section of a chamber is never taken apart.
05The other content areas
The T4 test has three content areas. Each has its own page of free questions:
The ASE T4 Brakes practice test 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.
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