T4 is the truck brake test: 60 questions (50 scored) in 75 minutes. Air brakes take 32 of the 50 scored questions, so this is mostly an air brake test with hydraulic and ABS sections attached. Some prep sites print "Air Brakes (64 questions)"; 64 is the percentage, not the count.
50 scoredquestions that count
75 mintesting time
25questions on the recert test
15practice questions here
What T4 covers
Checked Oct 23, 2026 against each source linked in the table. Content areas and scored questions from the spec table in the 2026 study guide, p. 22.
Technician A says the first reservoir downstream of the compressor and air dryer is usually the supply (wet) tank. Technician B says the air dryer is installed after the primary reservoir so that it receives cooled air. Who is right?
Explanation
Correct: The supply (wet) tank is the first reservoir after the compressor and dryer and collects most of the remaining moisture; the dryer sits ahead of it, not after the primary reservoir.
Incorrect: The air dryer is installed between the compressor and the supply tank so it dries air before any reservoir, so Technician B is wrong.
Incorrect: Only Technician A is right; the dryer comes before the reservoirs, not after the primary tank.
Incorrect: Technician A is right: the supply (wet) tank is the first reservoir after the compressor and dryer.
A tractor-trailer is fully charged, the engine is stopped, and the driver holds a full service application. After pressure stabilizes, the maximum acceptable air loss is:
Explanation
Incorrect: 6 psi per minute is more than the 4 psi per minute allowed for a combination with brakes applied.
Incorrect: 2 psi per minute is the released (no brakes applied) limit for a single vehicle.
Incorrect: 3 psi per minute applies to a single vehicle with brakes applied, or a combination with brakes released.
Correct: With the engine off and a full service application held, a combination vehicle must not lose more than 4 psi per minute once pressure settles.
One drive-axle brake with an automatic slack adjuster shows excessive applied stroke. Technician A says a loose brake chamber mounting could be the cause. Technician B says worn linings are the likely cause, because an automatic adjuster cannot compensate for lining wear. Who is right?
Explanation
Correct: A loose chamber mounting lets the chamber shift when the brake is applied, adding pushrod stroke; Technician B is wrong because taking up lining wear is the automatic adjuster's job.
Incorrect: An automatic slack adjuster exists to compensate for lining wear, so worn linings alone should not cause excessive stroke; Technician B is wrong.
Incorrect: Only Technician A is right; an automatic adjuster does compensate for lining wear.
Incorrect: Technician A is right: a loose chamber mounting can add to applied stroke.
With the parking brakes released and the service brakes not applied, air leaks from a drive-axle spring brake chamber. Technician A says a ruptured service diaphragm could be the cause. Technician B says a ruptured spring brake diaphragm could be the cause. Who is right?
Explanation
Incorrect: The service section holds air only during a service application, so a torn service diaphragm would not leak with the service brakes off; Technician A is wrong.
Correct: With the parking brakes released, hold-off air stays under the spring brake diaphragm all the time, so a ruptured spring diaphragm leaks continuously.
Incorrect: Only Technician B is right; the service diaphragm is not pressurized when the service brakes are off.
Incorrect: Technician B is right: a ruptured spring brake diaphragm leaks whenever the parking brakes are released.
The compressor discharge line on a truck with slow build-up is found partly blocked with carbon. Which of these could have caused the carbon buildup?
Explanation
Correct: A compressor that runs loaded most of the time because of air leaks runs hot and passes more oil, which bakes into carbon in the discharge line.
Incorrect: A purge at each governor cut-out is how the air dryer is meant to work.
Incorrect: A 100 psi cut-in is the federal minimum for trucks, a normal setting that does not cause carbon.
Incorrect: A discharge line of the specified length and size cools the air as designed; it does not create carbon.
A governor test shows cut-out at 142 psi and cut-in at 118 psi. The manufacturer's specification is cut-out at 120–130 psi. Which of these is indicated?
Explanation
Incorrect: The cut-out is 12 psi above the top of the spec, so the readings are not within it.
Incorrect: The 118 psi cut-in is above the 100 psi federal minimum for trucks.
Incorrect: The safety valve protects the system at about 150 psi; resetting it to hide a governor fault is wrong.
Correct: A 142 psi cut-out is over the 120–130 psi spec, so the governor must be adjusted or replaced.
A tractor has a relay valve for the drive-axle brakes. Technician A says that on release, the drive-axle chamber air exhausts through the foot valve exhaust port. Technician B says that during an application the foot valve sends a control signal to the relay valve. Who is right?
Explanation
Incorrect: Drive-axle chamber air exhausts through the relay valve's own exhaust port, close to the chambers, not back through the foot valve, so Technician A is wrong.
Correct: The foot valve sends a small control (signal) pressure to the relay valve, which applies the chambers with air from a nearby tank; on release, that air exhausts at the relay valve.
Incorrect: Only Technician B is right; a relay valve exists so the chamber air exhausts locally instead of traveling back to the foot valve.
Incorrect: Technician B is right: during an application the foot valve sends a control signal to the relay valve.
After several hard stops on a long downgrade, a truck's hydraulic brake pedal becomes soft and low; it returns to normal after the brakes cool. The most likely cause is:
Explanation
Correct: Brake fluid that has absorbed water boils at a lower temperature; after hard stops it boils in the calipers or wheel cylinders, and the vapor makes the pedal soft and low until it cools.
Incorrect: No pushrod free travel keeps the brakes from fully releasing and makes them drag, not a pedal that goes soft when hot.
Incorrect: Glazed linings cause brake fade with a hard, high pedal, not a soft, low one.
Incorrect: A worn primary cup lets the pedal sink at any temperature, not only after the brakes get hot.
When the parking brake knob is pushed in, the drive-axle spring brakes take much longer than normal to release; service brake operation is normal. The cause could be a:
Explanation
Incorrect: A sticking foot valve primary piston would affect service braking, which works normally.
Incorrect: A leaking service quick release valve affects service applications, not spring brake release.
Incorrect: A 100 psi cut-in is the federal minimum for trucks, a normal setting.
Correct: Spring brakes release when air flows through the spring brake control (relay) valve to the chambers; a sticking valve delays that flow, while the service brakes stay normal.
A tractor's compressor loads and unloads far more often than normal, and the tanks contain water. Static leakage is within limits, but air escapes from the dryer purge port while the compressor is pumping. The technician should:
Explanation
Incorrect: Drain valves are not the cause; static leakage is within limits.
Incorrect: Static leakage is within limits, so nothing points to the compressor or discharge line.
Correct: Air escaping from the purge port while pumping means the purge valve is not sealing; the compressor runs more and wet air passes on, so the purge valve is repaired or replaced.
Incorrect: Changing the governor range does not stop the leak at the purge port.
Answer every question to see your score
Area by area
Air brakes
32 scored questions. The supply side covers build-up and leakage tests, the governor and unloader, the air dryer, reservoirs and drains, the foot valve, relay and quick-release valves and the tractor protection valve. The foundation side covers chambers, automatic slack adjusters, S-cams, air disc calipers, linings, drums and wheel bearings. Spring brakes are applied by the spring and released by air, so losing air applies them.
Hydraulic brakes
11 scored questions: the master cylinder and bleeding, metering and proportioning valves, calipers and wheel cylinders, drums and rotors, fluid type, driveline parking brakes, and hydraulic or electric power assist. The car brake test, A5, stops here: air brakes are a T4 topic.
ABS, ATC, ESC & collision warning
7 scored questions: ABS self-test and warning lamps, blink codes and scan tool diagnosis, traction and stability control sensors (wheel speed, yaw, steering angle) and collision warning sensors with their calibration. FMVSS 121 requires ABS on single-unit air-braked trucks built since March 1, 1998.
FMVSS 121 or 49 CFR 393: two rulebooks
Two sets of federal numbers turn up in T4 questions. FMVSS 121 is the standard a new air-braked vehicle is built to; Part 393 of the Federal Motor Carrier Safety Regulations sets what a truck in service must meet.
Low-air warning: FMVSS 121 requires it below 60 psi. Section 393.51 holds trucks built since March 1, 1975 to that standard.
Lining thickness (393.47): steer-axle air drum 3/16 in for a continuous strip (1/4 in for two pads), other air drums 1/4 in, air disc 1/8 in, hydraulic 1/16 in.
Pushrod stroke (393.47): the readjustment limit depends on chamber type and on long-stroke design; a chamber not in the table is held to 80% of its rated stroke.
Automatic slack adjusters: one that keeps going past the stroke limit is a symptom of a fault in the adjuster or the foundation brake. Adjusting it by hand only hides the fault until the brake goes out of adjustment again.
Numbers from the eCFR text, read Oct 23, 2026.
60 psi or 55 psi?
Both numbers appear in brake material. 60 psi is the FMVSS 121 warning point that trucks built since 1975 meet. The "55 psi or half of governor cut-out" in 393.51 is for older power units that FMVSS 121 did not cover. Read which rule the question names.