ASE A5 cheat sheet

DOT 3, DOT 4, and DOT 5.1 fluids are glycol-ether based and hygroscopic; DOT 5 is silicone based and must never be mixed with glycol fluids because seal swelling and system damage result.

A pedal that sinks slowly to the floor under steady pressure with no visible external leak points to an internal bypass leak past the master cylinder cup seals.

Bleed order on a typical four-wheel system is farthest-first: right rear, left rear, right front, left front.

A residual pressure valve of about 10 psi is used with drum brakes to keep wheel cylinder cups seated; disc systems use about 2 psi or no valve at all.

A metering (hold-off) valve delays front disc application until rear drum pressure reaches roughly 100 psi, limiting nose dive on light stops.

A proportioning valve reduces rear hydraulic pressure at high apply pressure to delay rear wheel lockup.

Fluid that has absorbed moisture has a sharply lower boiling point; wet boiling point for DOT 4 is about 155 C (311 F), so old fluid fades under heat.

An overlong or misadjusted booster pushrod covers the master cylinder compensating port, trapping expanding fluid and causing brake drag and heat buildup.

Pedal feel diagnostics: spongy pedal means air in the hydraulics, pulsating pedal means rotor thickness variation, and a low pedal means rear shoe adjustment or air.

Duo-servo drum brakes are self-energizing: the primary shoe's rotation is transferred through the star-wheel adjuster to the secondary shoe, multiplying apply force.

Maximum drum inside diameter is stamped on the drum; a drum worn or machined beyond that dimension must be replaced.

Measure drum out-of-round and taper with an inside micrometer or dial bore gauge; out-of-round produces pulsation and uneven shoe contact.

On riveted linings, replacement is required at about 1/32 in (0.8 mm) above the rivet heads; bonded linings are replaced at the manufacturer's minimum spec.

A leaking wheel cylinder contaminates the linings with fluid, causing pull, grab, or loss of friction; service both sides as a pair and replace contaminated shoes.

On many systems the self-adjuster actuator lever rotates the star wheel during reverse stops to take up excess shoe-to-drum clearance.

A broken hold-down spring, return spring, or adjuster spring can cause dragging, grabbing, noise, or loss of self-adjustment.

Brake pull to one side commonly traces to a seized wheel cylinder, contaminated lining, or internally restricted hose on the affected side.

The star wheel is turned in the direction the actuator lever moves to expand the shoes; adjust until light drag is felt, then back off per specification.

Minimum rotor thickness is stamped on the rotor hat or hub; a rotor machined or worn below that value must be replaced, not resurfaced again.

Lateral runout is measured with a dial indicator perpendicular to the friction surface; typical OEM limit is about 0.002 to 0.004 in (0.05 to 0.10 mm).

Disc thickness variation causes a pulsating pedal without steering wheel shake; measure rotor thickness at eight or more points around the circumference.

The square-cut caliper piston seal both seals pressure and twists to retract the piston slightly when pressure releases, setting pad clearance.

Sliding calipers need clean, lubricated guide pins with high-temperature silicone grease; worn bushings or dry pins cause tapered pad wear and drag.

Do not push contaminated caliper fluid back through the ABS hydraulic unit; open the bleeder screw while retracting the piston with a clamp.

Squeal and rattle are controlled by shims, anti-rattle clips, and correct pad-to-caliper grease; missing hardware causes both noises.

Check hub flange runout before machining a rotor; an on-car lathe corrects runout in the assembled position and is used when hub runout is within spec.

Uneven inner-versus-outer pad wear indicates a sticking piston, while taper wear across one pad indicates a binding caliper slide.

Engine vacuum at idle is typically 18 to 22 inHg; the booster check valve holds that vacuum after the engine is shut off for reserve assist.

Operational test: with the engine off, pump the pedal several times to exhaust vacuum, hold the pedal, then start the engine; the pedal should drop slightly.

Leak test: run the engine, hold the pedal, then shut the engine off; the pedal should hold position for about 30 seconds.

A hard pedal with an otherwise normal-running engine points to a failed booster, a leaking check valve, or a disconnected or restricted vacuum hose.

Hydro-boost uses pressure from the power steering pump, typically over 1,000 psi, and stores reserve pressure in a nitrogen-charged accumulator.

On a hydro-boost system, a hard pedal after several stops with the engine off is normal once the accumulator reserve is depleted.

Booster pushrod length is checked with a gauge; too long a rod holds the master cylinder off its rest position and causes brake drag.

Diesel and some gasoline direct-injection engines use a belt- or cam-driven vacuum pump because manifold vacuum is too low for booster operation.

Tapered roller wheel bearings are set to about 0.001 to 0.005 in endplay by adjusting the spindle nut; sealed hub units are simply torqued to specification and are not adjustable.

A failing wheel bearing often changes noise when the vehicle turns because the loaded side gets louder; also check for play at 12 and 6 o'clock and at 3 and 9 o'clock.

Excessive hub flange runout creates rotor lateral runout and pedal pulsation even when the rotor itself is within specification.

Electric parking brake systems require service mode or scan-tool piston retraction before rear pad replacement; the piston must be reset afterward.

Drum-in-hat parking brakes use a small drum assembly inside the rear rotor hat and are adjusted through an access hole in the backing plate.

A misadjusted or sticking brake light switch can keep the brake lamps on, block cruise control engagement, and prevent shift interlock release.

A collapsed brake hose acts as a one-way valve, holding pressure at the caliper so the brake drags and the wheel overheats.

Uneven or over-torqued lug nuts distort the hub and rotor, producing lateral runout and thickness variation that shows up as pedal pulsation.

Tone rings and wheel speed sensors share the hub area with bearings; a damaged tone ring can trigger ABS activation at low speeds.

ABS components are wheel speed sensors, tone rings, the ABS control module, the hydraulic control unit with pump motor and solenoid valves, relay, fuses, and the warning lamp.

An illuminated ABS lamp means antilock function is disabled, but base hydraulic braking remains fully operational.

Inductive wheel speed sensors generate an AC sine wave and typically measure 800 to 1,500 ohms; active magnetoresistive sensors need a module-supplied voltage and output a digital square wave.

Sensor-to-tone-ring air gap is typically 0.02 to 0.05 in (0.5 to 1.2 mm); debris, a loose sensor, or a cracked tone ring causes dropouts or false activation.

Traction control uses the same wheel speed inputs plus throttle position; stability control adds steering angle, yaw rate, and lateral acceleration sensors.

Steering angle sensor calibration is required after an alignment or steering column/suspension repair on vehicles with stability control.

Air trapped in the HCU usually requires a scan-tool-actuated bleed sequence that cycles the pump and solenoid valves; pedal bleeding alone may not remove it.

ABS false activation at low speed is most often caused by a dirty or damaged tone ring, a loose sensor, or excessive air gap.

When the ABS pump does not run during an active test, check the pump motor relay and its fuse before condemning the hydraulic control unit.