ASE T4 Brakes (Medium/Heavy Truck) cheat sheet

Governor cut-out is typically 120-135 psi; cut-in occurs about 20-25 psi lower, restarting compressor delivery.

The low-pressure warning lamp and buzzer must activate at or below 60 psi system pressure.

The safety (pop-off) valve is set near 150 psi and protects the wet tank if the governor fails to unload the compressor.

The compressor unloader holds the inlet valves open at cut-out, so the compressor turns without pumping air.

Air dryer purge occurs on governor cut-out; the purge valve blasts moisture and oil from the desiccant to atmosphere.

A one-way check valve between the dryer and wet tank prevents reservoir air from bleeding back through a failed compressor.

Leak-down test: engine off, brakes fully applied, loss should not exceed about 3 psi/min on a single vehicle at stable pressure.

In a dual-circuit foot valve, each circuit has its own supply and delivery; loss of one circuit still leaves the other braking.

Applying air pushes the pushrod out, rotating the slack adjuster and S-cam; the cam lobes force the shoes outward against the drum.

Pushrod stroke limits depend on chamber size and type; a clamp-type Type 30 chamber is limited to 2 inches, a Type 24 clamp to 1 3/4 inches.

Automatic slack adjusters correct stroke by ratcheting on the return stroke; they must be checked at full application pressure for excess free stroke.

Install the slack adjuster so it sits at about 90 degrees to the pushrod at mid-stroke for maximum lever mechanical advantage.

Lining worn below about 1/4 inch at the shoe center, or contaminated with oil or grease, requires replacement.

The maximum inside diameter is stamped on the drum; turning or wear beyond it reduces lining contact and heat capacity.

A ruptured chamber diaphragm produces a constant air leak, weak or no braking at that wheel, and slow system pressure buildup.

A broken or missing shoe return spring causes the shoes to drag, overheating the drum and rapidly wearing the lining.

Wheel speed sensors are variable-reluctance units that generate an AC signal whose frequency rises with wheel speed.

The ABS modulator valve has three states: apply (hold-off), hold, and release (exhaust).

Sensor-to-tone-ring air gap is typically 0.010-0.040 inch; debris, a bent sensor, or a damaged tone ring causes intermittent faults.

During a normal stop without wheel slip, ABS stays passive and full service air pressure reaches the chambers.

The ABS lamp illuminates briefly at key-on as a bulb check and stays on when an active fault is stored; blink codes are retrieved per manufacturer.

ATC limits engine torque and applies drive-wheel brakes when the ECU detects acceleration spin, improving traction.

ABS does not shorten dry-pavement stopping distance in most cases; its primary benefit is maintaining steering control during hard braking.

A tone ring with missing or damaged teeth will set a wheel speed fault as soon as the vehicle moves.

DOT 3, DOT 4, and DOT 5.1 fluids are glycol-based and mutually compatible; DOT 5 is silicone and must never be mixed with them.

Glycol brake fluid is hygroscopic; absorbed moisture lowers boiling point and promotes corrosion and a spongy pedal.

A spongy or low pedal usually means air in the system; bleed starting at the wheel farthest from the master cylinder.

Residual pressure valves maintain about 10 psi in drum systems and 2 psi (or none) in disc systems to keep cup seals seated.

The metering valve delays front disc application briefly so rear drum shoes can overcome return spring force in disc/drum systems.

The proportioning valve limits rear hydraulic pressure during hard stops to reduce rear-wheel lockup and preserve stability.

A pedal that slowly sinks to the floor under steady pressure indicates an internal master cylinder cup seal bypass.

Brake pull to one side points to a sticking caliper, collapsed hose, or contaminated lining on that side.

Spring brakes apply mechanically when air is exhausted from the spring chamber and release when air pressure is admitted.

The yellow parking control valve exhausts spring chamber air to apply the parking brake; pushing it in admits air to release.

The inversion valve provides a modulated spring brake application when service brake pressure is lost, allowing a controlled emergency stop.

Caging bolts compress the power spring so a vehicle can be towed or moved; the spring brakes are inoperative while a bolt is installed.

Spring brake chambers are sealed units and must never be disassembled; the compressed power spring stores lethal energy.

The anti-compounding valve prevents service air pressure and spring force from applying simultaneously and overloading the foundation brake.

Per FMVSS 121, the parking brake system must hold a fully loaded vehicle on a 20 percent grade.

A DD3 chamber uses a mechanical lock instead of a stored spring and includes its own anti-compounding valve.

Typical wheel bearing end play is 0.001-0.005 inch; too little causes overheating, too much allows seal leakage and tire wear.

The seal lip rides on the spindle seal journal; a worn groove or scratch there causes repeat leaks even with a new seal.

Over-tightening the adjusting nut causes bearing overheating, roller spalling, and premature failure.

Wet hubs use gear oil and must not be mixed with grease; use the lubricant type specified for the hub design.

The cone is the inner race with rollers and the cup is the outer race pressed into the hub; replace them as matched sets.

Bearing spalling (flaking or pitting) results from fatigue, contamination, or inadequate lubrication and produces noise and vibration.

Drive a new seal squarely with the proper driver until it seats; a removed seal must never be reused.

Hub oil leaking onto brake linings requires lining replacement, not cleaning, because contaminated lining cannot be restored.