T4 Brakes — practice test

High-yield T4 topics

The material that shows up year after year. If you're short on time, start here.

• Air supply control numbers, cold. Governor cut-out 120-135 psi, cut-in about 20-25 psi below that, low-air warning must come on at or before 60 psi, spring brakes apply somewhere in the 20-45 psi window, and a healthy compressor builds 85 to 100 psi in roughly 45 seconds. Leak-down limits are their favorite: engine off, brakes released, 2 psi/min single and 3 psi/min combination; brakes applied, 3 and 4. If you have to think about which number goes where, you will lose easy points.
• Pushrod stroke and slack adjusters. Measure at 90-100 psi full apply with the engine off, not at whatever is in the tank. Know the readjustment limit vs the legal max for a standard Type 30 (about 1-3/4 in, 2 in max) and that a long-stroke 30 gets you to about 2-1/2 in. Long-stroke chambers wear a square air port and a trapezoidal or notched ID tag, and mixing chamber types across an axle is a violation. Out-of-adjustment brakes are the number one CVSA out-of-service item, so ASE tests it hard. You also want the slack arm near 90 degrees to the pushrod at full apply, because past over-center the mechanical advantage falls off a cliff.
• The valve tree and what each one does when it fails. Relay valve cracking around 4 psi and controlling rear timing, quick-release on the steers, tractor protection valve plus the red octagon trailer supply and yellow diamond park knob, spring brake modulating/inversion, ratio and limiting valves. Expect symptom-to-valve questions: trailer brakes lag or drag, one axle applies late, the trailer supply pops out on a hard apply. Red gladhand is emergency/supply, blue is service, and crossed lines is a classic scenario.
• Air dryer and compressor duty cycle. The dryer purges every time the governor unloads; desiccant saturation and a dead purge valve dump water into the tanks, and a bad heater freezes it in winter. But oil in the air is a compressor problem - rings, turbo oil supply, or a compressor running loaded more than about 25 percent duty cycle because of a leak somewhere else. Also know the discharge line has to slope down with no traps or it cokes shut.
• ABS on air trucks. Wheel speed sensors are push-adjust: shove them in until they touch the tone ring, then let the ring back them off to roughly 0.005-0.035 in. Spin the wheel by hand and you should see at least about 0.2 volts AC; resistance runs to OEM spec, generally hundreds to a couple thousand ohms. Know exciter ring tooth damage, that a hub with excessive end play fakes an intermittent sensor fault, and where the trailer ABS lamp lives (driver side, checked at key-on).
• Wheel bearings by TMC RP 618, not by feel. Torque the adjusting nut to about 200 lb-ft while rotating the hub to seat it, back it off completely, retorque to roughly 50 lb-ft, then back off to the specified fraction of a turn and lock it. End play target is 0.001 to 0.005 in checked with a dial indicator on the hub. Also know oil hub fill level and sight glass, unitized hubs that are not adjustable at all, and that a leaking wheel seal ruins the lining and gives you a pull.

Where techs lose points on T4

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• "Long stroke, adjust the slack." You cannot manually adjust an automatic slack adjuster to fix long stroke, and on the test that answer is always the bait. Long stroke on an ASA means something is worn or failed - clevis pin, cam bushings, cracked drum, dragging chamber return, or the adjuster itself. Manually cranking it in just masks the fault until it walks back out.
• Confusing the 60 psi low-air warning with the 20-45 psi spring brake application. They are two different events, and ASE writes scenarios where the driver reports the buzzer at highway speed or the parking brakes popping on. Same test, different systems - warning device vs spring brake pressure. Know which one the symptom points to.
• Blaming the dryer for oil, or the chamber for a pull. Oil in the tanks means the compressor is passing it or is running way over duty cycle because of an air leak elsewhere; the saturated dryer is downstream evidence. Same logic on a pull: the answer is usually a grease-contaminated lining, a timing/crack pressure mismatch, or a seized air disc guide pin, not the chamber on the side that is grabbing.
• Doing the leak test wrong in your head. Released and applied have different limits, and single vehicle and combination have different limits. Four numbers, and the question will tell you which condition it is in. Also: never disassemble a spring brake chamber. There is something like 1,800 to 2,000 lb of spring in there, and "cut it open to inspect" is a wrong answer that is also a way to get killed.

A study plan that works for T4

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-4: Own the air supply and control side. Compressor, governor, dryer, reservoirs, and every valve in the tree. Make a one-page number sheet (cut-out/cut-in, 60 psi warning, 20-45 psi spring apply, 2/3 and 3/4 leak-down, 85-100 in 45 sec) and rewrite it from memory every morning until it is automatic.
2. Days 5-8: Foundation and parking brakes. Stroke measurement procedure and limits, standard vs long-stroke ID, automatic slack adjuster diagnosis, S-cam and bushings, drum discard diameter, CVSA lining limits (1/4 in drum, 1/8 in air disc), air disc guide pins and tapered pad wear. Go out to a real truck and actually measure stroke on all six with a ruler at 90-100 psi.
3. Days 9-11: The smaller blocks that people skip and then lose by three questions. Hydraulic system (DOT 3/4 vs DOT 5 and why silicone stays out of ABS), Hydro-Max full-power boost with its electric backup, metering/proportioning, ABS/ATC, and wheel bearing adjustment straight out of RP 618. This is maybe a third of the test and it is the cheapest third to fix.
4. Days 12-14: Practice questions only, and rework every miss until you can say out loud why the wrong answer looked right. Drill the two-tech format specifically. If you are missing anything by more than one or two, it is a numbers problem, not a knowledge problem - go back to your sheet.

Sample T4 questions

Straight from the bank — answers highlighted, with the explanation underneath.