Your FREE ASE T4 Flashcards 2026 – 200+ Cards

A brake system that uses compressed air as the working fluid. Standard on most medium and heavy trucks — distinct from car hydraulic brakes.

The engine-driven pump that builds compressed air for the brake system. Loads (builds air) and unloads (idles) on command from the governor.

Senses reservoir pressure and cycles the compressor — unloads it at cut-out (~125 psi) and reloads it at cut-in (~100 psi).

The lower setpoint, about 100 psi, at which the governor returns the compressor to building (loading) air.

The upper setpoint, about 125 psi, at which the governor unloads the compressor so the system does not over-pressurize.

At cut-out it sends air to the compressor unloader to stop pumping; at cut-in it removes that signal so the compressor resumes pumping.

The mechanism the governor signals at cut-out to hold the intake valves open so the compressor stops building air.

Removes moisture and oil aerosols from the compressed air with a desiccant cartridge before it reaches the reservoirs, then purges the contaminants at cut-out.

The replaceable element inside the air dryer that absorbs water vapor from the compressed air.

A spring-loaded relief valve, usually on the wet tank, that vents excess pressure near 150 psi if the governor fails to unload the compressor.

Stores compressed air. Trucks have a wet (supply) tank plus primary and secondary service tanks.

The first tank after the compressor; it collects moisture and oil and is drained regularly. Feeds the primary and secondary tanks.

The two service tanks of a dual air system, each feeding a separate brake circuit so one failure leaves braking on the other circuit.

Lets air into a reservoir but not back out, isolating each tank so a leak in one circuit cannot drain the others.

The valve at the bottom of each tank used to drain accumulated moisture and oil; some are automatic.

Gives the driver a continuous warning (light and buzzer) before service reservoir pressure falls below 60 psi, as required by FMVSS 121.

The pressure switch that closes the warning circuit when air drops below the threshold. If the warning never comes on, suspect this switch (or the buzzer/lamp).

Federal rule: the low air pressure warning must give a continuous signal before service reservoir pressure falls below 60 psi.

The driver-operated service brake valve that meters air to the chambers in proportion to pedal travel, usually controlling primary and secondary circuits separately.

The air pressure the treadle valve delivers to the chambers, rising in proportion to how far the pedal is pressed.

A valve near the rear chambers that uses the small treadle signal to deliver full reservoir air quickly to the rear brakes, speeding application and release on long trucks.

Typically at the front chambers; lets chamber air exhaust locally on release instead of traveling back to the treadle valve, speeding brake release.

The actuator that converts air pressure into mechanical force: air pushes a diaphragm and pushrod that move the slack adjuster.

The flexible rubber membrane inside the chamber that air pressure pushes against to extend the pushrod. A leaking diaphragm reduces braking force.

A common chamber size; a standard clamp-type Type 30 has an applied pushrod stroke (readjustment) limit of about 2 inches.

A chamber that allows slightly more stroke (about 2.5 inches on a Type 30) before being out of adjustment; identified by markings or a trapezoidal port.

The lever that converts brake-chamber pushrod motion into rotation of the S-cam shaft, applying the foundation brake and setting lining-to-drum clearance.

Adjusted by turning the adjusting bolt to take up clearance until the shoes contact the drum, then backing off to a free-rolling clearance.

Maintains correct lining-to-drum clearance automatically. One that needs frequent manual adjustment is defective and should be replaced.

How far the chamber pushrod travels during a full application. Excessive stroke means the brake is out of adjustment and cannot make full force.

Chock the wheels, build full air, mark the pushrod at rest, make a full application (~90–100 psi), and measure how far the mark moves.

The S-shaped cam rotated by the slack adjuster that spreads the two brake shoes against the drum. The most common heavy-truck foundation brake.

The chamber pushrod extends, rotates the slack adjuster and camshaft, and the S-cam lobe spreads the two shoes outward against the drum.

The mechanical wheel-end braking assembly — S-cam drum, wedge, or air disc — that creates the friction to slow the wheel.

A drum brake in which the chamber drives a tapered wedge between plungers to spread the shoes against the drum; often self-adjusting.

A heavy-truck disc brake actuated by air through a caliper instead of an S-cam and drum; offers fade resistance and easier inspection.

The friction parts forced against the drum by the S-cam. Worn linings increase chamber stroke and lengthen stopping distance.

Rotates with the wheel; friction from the shoes slows it. Excessive wear or the wrong friction material causes scoring and heat.

Support the camshaft. When worn, the S-cam is not seated correctly, causing uneven shoe wear and a wheel that needs more pressure to brake.

Pulls the brake shoes away from the drum on release. A broken return spring causes drag and uneven wear.

The shaft carrying the S-cam, rotated by the slack adjuster. Excessive end play affects brake action.

A tandem (double-diaphragm) chamber that adds a powerful internal spring providing parking and emergency braking when air is released.

Applies mechanically by spring force when air is exhausted from the spring section. Failsafe — it cannot leak off and applies during major air loss.

Air is exhausted from the spring side, letting the caged power spring extend and mechanically apply the foundation brake. No air needed to stay applied.

Mechanically compressing the power spring (with the caging bolt) to release the brake for service or towing. Always cage before disassembly — stored force is deadly.

Air RELEASES a spring brake (caging the spring); loss of air APPLIES it. The reverse of how the service brake works.

Applies the spring brakes when system air drops below a safe level. A spring brake that does not apply automatically at low pressure points first to this valve.

The dash valve (yellow knob) the driver uses to apply and release the tractor spring (parking) brakes.

Automatically seals off the trailer air lines and exhausts the trailer supply line when pressure drops dangerously (~20–45 psi) or the trailer breaks away.

The dash valve that charges the trailer air system; it pops out automatically if trailer supply pressure is lost, applying the trailer spring brakes.

The interlocking coupling that joins the tractor and trailer air lines — one for the service (control) line, one for the supply (emergency) line.

The service (control) line carries the braking signal; the supply (emergency) line carries air to charge and release the trailer brakes. Don't cross-connect the gladhands.

When the trailer separates and the supply line is severed; the tractor protection valve isolates the tractor and the trailer spring brakes apply automatically.

Selects the higher of two air pressure sources to feed a circuit; used for dual-circuit control and anti-compounding.

Prevents the spring and service brake forces from adding together on the same chamber, which could over-stress the foundation brake. Done via a double-check valve.

Closes off an accessory or secondary circuit when pressure drops, preserving air for the primary braking circuits.

A check of the treadle valve's delivery and return; a slow return to cut-out pressure suggests a faulty full-function valve.

Measures air loss with the brakes applied. Excessive loss points to leaking service brake chambers or fittings.

With the engine off and brakes released, measures pressure drop over time; a federal limit governs allowable leakage per minute.

Times how long the system takes to build from a low pressure to cut-out; slow build-up indicates a compressor, intake, governor, or leak problem.

Worn compressor, clogged intake filter, restricted lines, leaking governor, or a system air leak.

The compressor cutting in and out too often; the most common cause is an air leak somewhere in the system.

Usually caused by a leaking air compressor oil seal letting oil pass into the discharge air — not a dryer fault.

With the trailer connected and the system charged, a steady leak there indicates a defective tractor protection valve.

Suspect a faulty quick-release or relay valve, a restricted line, or over-adjusted brakes — air is not exhausting from the chambers fast enough.

Often caused by unevenly adjusted slack adjusters, so the brakes apply at different times across the axles.

Can be caused by brake chambers of different sizes side-to-side, a stuck wheel-end, or grease-contaminated lining on one side.

On application, often indicates worn brake linings, since the chamber must extend farther to apply the same force.

Loss of braking from overheated drums and lining (mechanical fade) — a reason proper adjustment, lining, and engine/auxiliary braking matter on long grades.

If air rises too slowly between setpoints, inspect the governor cut-in and cut-out settings that control when the compressor loads and unloads.

An extremely hot compressor discharge line can be caused by a faulty compressor discharge valve creating back pressure.

Two independent service circuits (primary and secondary) so a failure in one still leaves braking on the other.

Compressed air does the work instead of the driver's leg, allowing strong, reliable braking on large vehicles and a failsafe spring-brake design.

Compressor builds air → governor regulates → dryer cleans → reservoirs store → treadle/relay valves meter → chambers operate the foundation brakes.

Improper contact after an S-cam or shoe replacement can cause squealing; check seating and arc of the lining first.

The first thing to inspect when an air-braked truck takes longer than usual to stop — worn linings greatly increase stopping distance.

Often results from using brake linings with the wrong friction material, increasing abrasion and heat.

Incorrectly adjusted or damaged wheel speed sensors can trigger ABS during normal braking on an air-braked truck.

When the compressor cycles too often, inspect the whole air system for leaks before condemning components.

If one wheel needs more air pressure to apply than the others, the S-cam may be incorrectly seated in the bushings.

A significant air-pressure drop after shutdown points first to a failing one-way check valve or a reservoir/valve leak.

Checks for leakage at the treadle (foot) valve; a drop after a rebuild may indicate improper installation of the valve.

On a tandem axle, a likely cause is a clogged exhaust port in the spring brake modulating valve trapping air on the spring side.

Typically relieves around 150 psi, well above cut-out (~125 psi), to protect the system if the governor fails.

The pushrod and slack adjuster should form roughly a right angle at full application for proper leverage; misalignment reduces braking force.

Connect the brake chamber pushrod to the slack adjuster. Seized or worn clevis pins cause poor brake action and false free-play readings.

More than about one inch of pushrod free play often indicates incorrect initial slack-adjuster setup.

A clogged compressor intake filter restricts air flow and slows pressure build-up even with a good compressor.

Even braking across axles; achieved with correct adjustment, matched chamber sizes, and proper valve timing (relay, limiting valves).

Older systems could limit steering-axle braking on slick roads; modern trucks brake all axles for stability. Know the function if it appears.

Service and supply gladhands are color-coded or marked (commonly blue service, red emergency) to prevent cross-connection.

Federal rules specify minimum reservoir volume relative to chamber displacement so several applications are possible without the compressor.

Limits how quickly the brakes must apply and release; relay and quick-release valves help meet these timing requirements.

Routine maintenance to remove accumulated water and oil; even with an air dryer, manual or automatic draining is still done.

When applied pushrod stroke exceeds the chamber's limit, the brake is out of adjustment and must be readjusted or repaired before service.

The governor is usually mounted on or near the compressor and connected to a reservoir; its line and setpoints are checked during air-system diagnosis.

A dash- or column-mounted lever (trolley valve) that applies only the trailer service brakes independently of the foot valve.

A trailer valve that applies the trailer brakes using reservoir air if the service signal or supply is lost.

Longer stroke reduces the mechanical advantage at the S-cam, so an out-of-adjustment brake makes less force at the same air pressure.

A one-way valve at the compressor head; if faulty it can cause high discharge-line heat or poor pressure build-up.

If there are no air leaks or mechanical faults, inspect the brake chamber diaphragm next, since it affects the force delivered.

A buzzer that sounds at normal pressures usually means a defective low-pressure switch signaling at the wrong time.

The relay valve on the trailer that speeds application and release of the trailer brakes using its own reservoir air.

The band securing the two halves of a clamp-type chamber. Never loosen it on a spring chamber without caging the spring.

Common air-brake component manufacturers; service procedures and specs follow the manufacturer's published values.

Verify each tank holds pressure independently; a failed check valve lets one circuit's leak drain another tank.

An auxiliary retarder that uses engine compression to slow the truck, reducing service-brake heat on grades. Not part of the air service brake itself.

Oil or grease on the lining reduces friction and can cause pulling or grabbing; the source (seal/leak) must be corrected.

Mounts the brake shoes and anchors; the spider locates the foundation-brake components at the axle end.

The passage that sends governor signal air to the compressor unloader; a stuck port can prevent proper loading/unloading.

Replacing worn shoes/linings; afterward, adjust the slack adjuster and verify equal stroke and lining-to-drum contact.

Charge the system, shut the engine off, release the brakes, and watch the gauge; pressure must not drop faster than the federal allowable rate.

Belt- or gear-driven from the engine; a slipping drive or worn gear reduces output and slows build-up.

Indicates the chamber's effective diaphragm area (e.g., Type 24, Type 30); larger numbers make more force and have their own stroke limits.

Air brakes don't lose pressure like a hydraulic boil-off; air fade is mainly mechanical (overheated drums/lining), which adjustment and proper lining limit.

With brakes applied, the slack adjuster should be near 90° to the pushrod; a bad angle signals adjustment or stroke problems.

Tanks must be secured and free of external corrosion; a corroded or loose tank is an inspection failure and a safety risk.

When the governor cuts out, the dryer purges collected water and oil to atmosphere; a failed purge valve lets moisture into the tanks.

Allow moisture out of the non-pressure side; blocked holes can trap water and corrode internal parts.

Spring brake chambers carry a caging tool/bolt; using it correctly is required to safely service the chamber.

Manual slack adjusters need periodic checking; automatic slack adjusters self-maintain but are inspected for stroke and replaced if out of spec.

Delivers metered air to two separate circuits; loss of one circuit still allows braking through the other.

Applying soap solution to fittings and valves reveals air leaks by bubbling — a standard method for locating system leaks.

Some chambers have a stroke-indicator marking so technicians can quickly judge adjustment during inspection.

A one-way check valve between the wet tank and the service tanks holds air in the service circuits if the supply side leaks down.

Springs, rollers, bushings, and anchor pins replaced during a brake job to restore correct shoe movement and return.

Verifying that the truck stops straight and within distance after repair, with no pulling, dragging, or warning lights.

Many compressors are lubricated by engine oil; a failed seal lets that oil into the air system (oil contamination).

An air-actuated pressure switch that closes the brake-light circuit when the treadle valve applies pressure.

Push the trailer supply (red) knob to charge the trailer; the system must build pressure before the trailer spring brakes release.

Out-of-adjustment brakes are among the most common roadside violations; correct stroke is a safety and compliance priority.

A hybrid where air pressure operates a hydraulic actuator at the wheels; combines an air supply with hydraulic foundation brakes.

Braking force depends on air pressure, chamber size, slack-adjuster length, S-cam radius, and lining/drum condition working together.

Dual gauges (or a split gauge) show primary and secondary pressures; the driver monitors both for a circuit failure.

Trapped moisture corrodes valves and can freeze and block lines in cold weather, so reservoirs are drained and the dryer is maintained.

The air pressure needed in the spring section to keep the parking brake fully released; below it, the spring starts to apply.

Service brakes are air-applied (foot valve); parking/emergency brakes are spring-applied and air-released — two distinct functions in one chamber.

Set the slack adjuster, verify equal stroke on both wheels of an axle, and confirm full lining-to-drum contact before returning to service.

A restricted or leaking governor signal line causes incorrect cut-in/cut-out and erratic compressor operation.

The 60 psi warning is an early alert; if pressure keeps falling, the spring brakes apply automatically as a failsafe.

Uses a master cylinder to pressurize brake fluid that applies the wheel brakes; often boosted by vacuum, hydroboost, or air-over-hydraulic power.

Converts pedal force into hydraulic pressure. Internal leakage causes a slowly sinking pedal with no external fluid leak.

Indicates air trapped in the system (air compresses, fluid does not). Fix by bleeding; air can hide in the ABS modulator.

With a full reservoir and no external leak, points to internal master-cylinder failure; test the master cylinder first.

Glycol-based, hygroscopic hydraulic fluid that absorbs water over time. Water lowers its boiling point and corrodes components.

A sign of water contamination. Flush and replace with fresh sealed fluid of the correct DOT rating.

Silicone-based, not hygroscopic, and NOT compatible with DOT 3/DOT 4 glycol fluids — never mix the two.

Purging air from the hydraulic system through the bleeder screws so the pedal is firm; required after opening the system.

Water in the fluid can flash to vapor under hard braking, causing a sudden soft pedal and brake fade.

Limits hydraulic pressure to the rear brakes to balance braking and prevent early rear lock-up. Misadjusted = rear brakes lock early.

When one wheel brakes poorly or locks, suspect a seized or sticking caliper piston at that wheel.

Uses engine vacuum to multiply pedal force. A hissing noise with reduced braking points to a faulty vacuum booster.

A hydraulic booster powered by the power-steering pump, used where engine vacuum is unavailable (common on diesels).

A hose that swells under pressure is weak or damaged and near failure; replace it.

A star-wheel adjuster maintains shoe-to-drum clearance; excessive pedal travel can mean the adjusters need attention.

Pushes the shoes against the drum in a hydraulic drum brake. A leaking wheel cylinder causes fluid loss and weak braking at that wheel.

Fluid loss with no visible leak is internal master-cylinder leakage; visible leaks come from cylinders, calipers, hoses, or lines.

A combination pointing to a wheel-end hydraulic fault (sticking caliper/wheel cylinder) plus air in the line on that circuit.

Replacing all old, moisture-laden fluid with fresh fluid; required when fluid is contaminated or at service intervals.

Filled to the top, fluid has no room to expand as it heats and pads wear, which can cause brake drag; fill to the marked line.

Lit by a pressure-differential switch when one hydraulic circuit loses pressure relative to the other.

After a pad/shoe change, check adjustment and that the pads are seated; air or worn parts also lengthen travel.

Maintains a small pressure in drum-brake circuits to keep wheel-cylinder cups seated and speed reapplication.

Clamps the pads against the rotor; piston seals retract the pad slightly on release. Sticking causes drag and uneven wear.

The disc the pads clamp; warping causes pedal pulsation, and scoring or thickness below spec requires machining or replacement.

An air-applied hydraulic cylinder that supplies the foundation brakes on some medium-duty trucks; combines air control with hydraulic apply.

Spongy = air; sinking = master-cylinder internal leak; hard pedal/low assist = booster fault — distinct symptoms with distinct causes.

Bleeding the master cylinder on the bench before installation removes trapped air and prevents a spongy pedal afterward.

Usually a separate mechanical (cable) drum or driveline brake, since hydraulic service brakes cannot hold by stored pressure.

Test strips or a meter measure water content; high moisture means the fluid should be flushed even if it looks clear.

Uses wheel speed sensors and a modulator to rapidly reduce, hold, and reapply brake pressure, preventing wheel lock-up so the driver keeps steering control.

The valve assembly that adjusts brake pressure to each controlled wheel during an ABS event, on command from the ABS controller.

Reads a toothed exciter (tone) ring to report wheel speed to the ABS. A damaged sensor or excess air gap sets a fault and can cause false ABS activity.

The toothed ring on the hub or axle the wheel speed sensor reads; missing or damaged teeth cause wheel-speed faults.

The small clearance between the wheel speed sensor and the tone ring; too large a gap weakens the signal and can trigger ABS faults.

Normal ABS operation — the system is rapidly modulating brake pressure to prevent lock-up. Not a fault.

Indicates an ABS self-test failure or stored fault; the base brakes still work but ABS may be disabled until repaired.

Usually means a disconnected or damaged wheel speed sensor breaking the electrical signal.

Caused by a misadjusted/damaged sensor, a damaged tone ring, or incorrect tire size giving false wheel-speed readings.

Mismatched tire sizes make wheels turn at different speeds, which the ABS can misread and act on during normal braking.

Controls pressure to an individual brake circuit during an ABS event, allowing precise per-wheel modulation.

Maintains steering control and stability; stopping distance can be longer than on dry pavement because of reduced traction.

Suggests a problem in the hydraulic ABS control unit, with unregulated pressure control.

Builds on ABS to limit drive-wheel spin during acceleration by braking the spinning wheel or reducing engine torque, improving traction.

Uses yaw and steering inputs to selectively brake individual wheels and cut power, countering rollover and loss-of-control (jackknife).

ATC is acceleration-focused (drive-wheel spin); ESC is stability-focused (rollover and jackknife in turns and emergencies). Both use ABS hardware.

Reports the tractor (and combination) ABS; it lights briefly at start-up as a self-test, then goes out if healthy.

A separate lamp, required on the trailer and visible to the driver, that reports the trailer's ABS — locating a trailer-only fault.

Means the ABS fault is in the trailer's system, not the tractor's.

Both ABS lamps illuminate briefly when the key is turned on, then extinguish if the systems pass their self-check.

Air ABS modulates air pressure at the chambers; hydraulic ABS modulates brake-fluid pressure. Both use wheel speed sensors and a controller.

Some ABS controllers flash fault codes through the warning lamp; a scan tool gives more detail for diagnosis.

Uneven brake wear on an ABS truck can trace to faulty wheel speed sensors or a damaged tone ring causing imbalanced response.

The controller that reads the wheel speed sensors and commands the modulator; it stores fault codes for diagnosis.

ABS reduces lock-up and preserves steering, but cannot beat physics — traction limits still set the shortest possible stop.

A subset of stability control that specifically reduces rollover risk by braking and cutting power when it senses high lateral force.

Creates a periodic speed-signal glitch the ABS may read as a wheel slipping, triggering unwanted ABS activity.

Many ABS faults are wiring-related; verify power, ground, and sensor circuits before replacing the controller.

ABS only modulates the existing brakes; out-of-adjustment air brakes or worn hydraulic brakes still must be fixed for the system to work well.

ABS, ATC, and ESC all rely on the same wheel speed sensors and modulator hardware, so a sensor fault can disable more than one function.