H4 Transit Bus — Brakes — practice test

High-yield H4 topics

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

• Air supply numbers, cold. Governor cut-out 120-135 psi with cut-in about 20-25 psi below it, build 85-100 psi in roughly 45 seconds, low-air warning on by 60 psi, and spring brakes auto-applying somewhere in the 20-45 psi window per FMVSS 121. Leakage: engine off, system charged, brakes released, no more than about 2 psi/min on a single vehicle and 3 psi/min applied. They will hand you a reading and ask what failed.
• Pushrod stroke and automatic slack adjusters. Know that a standard-stroke Type 30 clamp chamber carries a 2.0 in readjustment limit and the long-stroke 30 is rated 2.5 in but still gets flagged around 2.0, and that long-stroke chambers are identified by the square or trapezoidal air port and the tag. More important: an ASA out of stroke means the adjuster or the foundation is worn, not that it needs adjusting.
• Air disc brakes on transit. Bendix ADB22X and Knorr units are everywhere on low-floor buses. Uneven or tapered pad wear points at seized guide pins and torn boots, not a weak chamber. Know rotor minimum thickness and disc thickness variation, the caliper wear indicator, and that the adjuster is internal and self-adjusting - you verify with a running clearance check, you do not turn it to compensate for wear.
• ABS wheel speed sensors and tone rings. Sensor resistance in the 800-2000 ohm range, output around 0.2 V AC minimum when the wheel is spun by hand at roughly 30 rpm, and the setup is push the sensor in until it bottoms on the tone ring and let the hub back it off to gap. A cracked or debris-packed tone ring reads fine on the meter and only sets an intermittent code at road speed. Also know ABS reverts to normal braking and that an ABS event commands the retarder off.
• Retarders and the ABS handshake. Allison output retarder, driveline retarder, engine brake - know how each is commanded, that all are inhibited on wheel slip, and that a retarder that will not engage is usually a coolant temp limit, a service brake switch, or a J1939 request problem rather than a hydraulic fault. On a bus the complaint shows up as brake fade, not as a retarder complaint.
• Transit interlocks and wheel bearings. Brake-and-door interlock, fast idle, kneel and ramp interlocks: a bus that will not release at the stop is a door switch or interlock solenoid, not the foundation. On bearings use the TMC RP618 sequence (torque while rotating, back off, snug, back off a quarter turn) targeting 0.001-0.005 in endplay. Oil bath hubs get checked at the sight glass, and a weeping seal contaminates linings on that wheel end only.

Where techs lose points on H4

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

• Manually adjusting an automatic slack adjuster to bring stroke back in spec. It looks right, it will pass a stroke check that afternoon, and it is wrong on the test and in the shop. Manual adjustment on an ASA is for installation setup only.
• Oil in the air dryer, so replace the dryer. Wrong. Oil carryover means the compressor is passing oil or running past roughly a 25 percent duty cycle because the system is leaking or the governor line is restricted. Change the dryer without fixing the source and it comes right back.
• Swapping the two low-pressure numbers. Warning light and buzzer come on by 60 psi; spring brakes drag themselves on around 20-45 psi. A distractor will reverse them and it reads perfectly fine if you never memorized both.
• Bus pulls on application, so it must be a bad lining or a weak chamber. On air disc that is usually a seized caliper slide or a collapsed hose acting as a one-way restriction; on S-cam it is often unequal pushrod stroke side to side. Measure stroke on both wheel ends before you condemn anything.

A study plan that works for H4

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

1. Days 1-5: Air system end to end. Compressor, governor, dryer purge cycle, and every valve in the service circuit - relay, quick release, dual check, inversion, spring brake control. Drill the numbers (cut-out, cut-in, build, leak rates, warning, auto-apply) until you can recite them without thinking. Biggest block on the test and the cheapest points.
2. Days 6-9: Foundation and parking. S-cam stroke limits and long-stroke ID, then air disc - guide pins, rotor specs, running clearance, wear indicators. Spend real time on spring brake chambers, caging, and why you never split a piggyback. If there is a bus in the shop, run a full stroke check on every wheel end and write the numbers down.
3. Days 10-13: ABS, traction control, and retarders. Sensor specs, tone ring failures, modulator function, blink codes, and how ABS talks to the retarder and to engine torque over J1939. Fold the interlocks in here - door/brake, fast idle, kneel, ramp - because that is where the transit-only questions live and where most techs are cold.
4. Days 14-16: Hydraulic and bearings, then practice. Hydro-boost versus vacuum boost, Hydro-Max backup pump testing, DOT 3 versus DOT 5 and why you never mix them, TMC RP618 bearing procedure. Then run timed practice sets and review every miss down to the actual reason it was wrong, not just the letter.

Sample H4 questions

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