Hydraulics, power assist and parking brakes

Incorrect: A ruptured diaphragm cannot hold vacuum, so the pedal would not move down at start-up; a hiss and a hard pedal are its signs.

Incorrect: A restricted supply hose would give little or no pedal drop at start-up and a hard pedal while driving.

Correct: With vacuum used up and the pedal held, starting the engine restores assist, so a slight drop under the same foot force means the booster works.

Incorrect: A bypassing master cylinder lets the pedal keep sinking under steady force; a slight one-time drop at engine start is not that.

Incorrect: A faulty check valve shows up as lost reserve after the engine stops, not in this engine-start test.

Incorrect: A short pushrod adds free travel and a lower pedal; it never closes the compensating ports.

Correct: A pushrod that is too long holds the master cylinder pistons past the compensating ports, so heated, expanding fluid cannot return and all four brakes drag after a few miles.

Incorrect: Air in the lines makes the pedal spongy; it does not hold pressure on the brakes.

Incorrect: Sticking slide pins drag at the front calipers only; they cannot make the rear brakes drag too.

Incorrect: A worn piston seal lets pressure leak back, which causes a sinking pedal rather than drag.

Incorrect: Disconnecting the battery negative cable leaves the spindle where it is, so the piston still cannot be pushed back over the extended nut.

Incorrect: Prying the caliper piston back against an extended spindle can strip the nut or damage the motor gear set.

Incorrect: Applying the EPB with the pedal pressed extends the spindle and clamps the pads, the opposite of what is needed before service.

Correct: Entering EPB service (maintenance) mode with a scan tool makes the module drive the caliper motor back to its retracted position, so the piston can then be pushed in without fighting the spindle.

Incorrect: Unplugging the caliper motor also leaves the spindle extended, and many systems set a fault when the ignition is next turned on.

Correct: A 16 in. Hg loss between the pump outlet and the booster inlet can only happen in what connects them, so the hose or check valve in between is restricted or leaking.

Incorrect: Pushrod length affects free travel and drag; it has no effect on vacuum supply readings.

Incorrect: A diaphragm leak would pull both readings down together; a large drop between two points on the same line points to that line.

Incorrect: A master cylinder leak is hydraulic; it cannot change vacuum readings, and it would make the pedal sink rather than stay hard.

Incorrect: Worn vanes would lower the reading at the pump itself, but the pump outlet shows a healthy 21 in. Hg.

Incorrect: Wheel speed sensors feed ABS and auto-release logic; they do not set clamp force or explain a hold fault right after a pad job.

Correct: After new pads, the EPB module must relearn the caliper apply position (called teach-in, calibration or initialization depending on the maker); until it does, it flags a fault and may not clamp fully.

Incorrect: Nothing in the pad service touched the control module, so replacing it before the required relearn is a guess.

Incorrect: Caliper-mounted EPB units have no cable to adjust, and cable-type systems still need their electronic calibration after service.

Incorrect: A booster vacuum check valve belongs to vacuum-assisted brakes, which this vehicle does not use.

Incorrect: There is no vacuum booster on this system, so an engine vacuum hose does not supply its assist.

Incorrect: Power steering pump pressure drives a hydro-boost unit, a different assist design from an electric pump system.

Incorrect: Intake manifold vacuum matters only for a vacuum booster; an electro-hydraulic unit does not use it.

Correct: This system builds assist with an electric pump motor, so its power, ground and relay circuits are checked before the hydraulic unit is condemned.

Incorrect: A scan tool commanded ABS bleed removes air from the HCU, but it is a repair step, not a test that tells air from a bypassing master cylinder.

Incorrect: Moisture content tells when the fluid needs replacing; it does not separate trapped air from an internal leak.

Incorrect: An external leak test finds fluid escaping outside; neither trapped air nor an internal bypass leaks outside.

Correct: Plugging the outlets takes the lines and wheels out of the circuit: a pedal that goes firm proves the master cylinder holds and the softness is air downstream, while one that still sinks condemns the master cylinder.

Incorrect: An ultrasonic leak detector hears escaping gas; a liquid seep at a fitting makes no sound to pick up.

Incorrect: A smoke test finds vacuum and EVAP leaks; smoke cannot be pushed through a fluid-filled brake line.

Incorrect: Gravity bleeding and watching the bleeders puts almost no pressure on the fitting, so a seep stays hidden.

Correct: Pressure testing holds the hydraulic system at working pressure, which forces fluid out of a fitting that stays dry at rest and shows as pressure decay.

Incorrect: Vacuum-testing the brake booster checks the assist unit, not a hydraulic fitting.

Correct: Petroleum fluid swells every rubber seal and hose it reaches, and flushing cannot undo that, so all rubber parts are replaced and the system is flushed clean.

Incorrect: The master cylinder is only one part with rubber seals; calipers, wheel cylinders and hoses are contaminated too.

Incorrect: Draining and refilling the reservoir leaves contaminated fluid in the lines and swollen seals in place.

Incorrect: A flush removes the oil but leaves seals that are already swollen and will fail.

Incorrect: The contamination reaches every circuit, so fixing only the ABS modulator leaves swollen seals at the master cylinder and wheels.

Correct: Technician A is right: copper above about 200 ppm, the guideline used with copper test strips, shows the corrosion inhibitors are spent, so the fluid is flushed.

Incorrect: Technician B is wrong: clear fluid can still be depleted, and color does not reveal dissolved copper or moisture.

Incorrect: Both cannot be right, because B's 'it looks clear' reasoning contradicts the strip reading A relies on.

Incorrect: 'Neither' is wrong because Technician A's flush call matches the copper strip reading.

Incorrect: Clearing the code without repairing the corrosion only waits for the next rain to set it again.

Incorrect: A side-specific code that appears only in wet weather points to that corner's wiring, not to the shared control module.

Incorrect: A new EPB motor would plug into the same corroded connector, so the fault would return.

Incorrect: The motor winding tests within specification, so a new motor and caliper would leave the corroded connector in place.

Correct: The motor checks good at its own pins, so the high resistance is in the corroded terminals; cleaning or replacing them and sealing the connector stops the water entry that sets the code in rain.

Correct: An internally collapsed hose acts like a check valve: pressure reaches the caliper but cannot return, so the pads keep dragging after the pedal is released.

Incorrect: A scored rotor makes noise and wears pads faster, but it does not keep the caliper applied.

Incorrect: A low fluid level can let air in and soften the pedal; it cannot hold the brakes on.

Incorrect: Thin pads need more piston travel, but they do not keep pressure on the rotor.

Incorrect: Prying against the rotor pushes straight without turning the piston and can gouge the rotor or tear the boot.

Incorrect: A C-clamp only pushes straight in and cannot turn the piston, so it can damage the internal parking brake screw.

Incorrect: Push-back pliers retract ordinary pistons, but they cannot rotate one that threads in on a parking brake screw.

Correct: On a caliper with an integral mechanical parking brake the piston rides on a screw, so it must be turned while pressed in, which a wind-back tool does.

Incorrect: Engine oil also floats on glycol fluid, but it is amber or dark, not purple.

Incorrect: Water absorbed by DOT 4 darkens the fluid over time, but it dissolves into the glycol and does not form a purple layer.

Correct: DOT 5 silicone fluid is usually dyed purple and does not mix with glycol fluid, so it sits in a separate layer.

Incorrect: Power steering fluid is petroleum-based and does separate, but it is not purple, and it swells the seals.

Incorrect: DOT 5.1 and DOT 3 are both glycol fluids, so they mix completely and do not separate.

Incorrect: Engine temperature has its own gauge or lamp and does not switch the brake warning lamp.

Correct: The red brake lamp is switched by the reservoir float, so a low level (from a leak or worn linings) is the first and easiest check once the parking brake is off.

Incorrect: Battery voltage is not one of the basic inputs to the red brake lamp, so it is not the first check.

Incorrect: Low tire pressure lights the TPMS lamp, not the red brake warning lamp.

Incorrect: A metering valve only holds off the front discs at the start of a stop; it neither limits rear pressure nor switches the lamp.

Incorrect: A pressure differential switch only lights the warning lamp when the two circuits become unbalanced.

Correct: A combination valve combines all three jobs in one body: metering for the fronts, proportioning for the rears, and the warning-lamp switch.

Incorrect: A proportioning valve only limits rear pressure in hard stops; the other two jobs belong to other sections.

Incorrect: Lubricating a cable eases sticking, but it cannot repair broken strands.

Incorrect: Taping over the frayed strands hides the damage and leaves a weak cable that can snap or snag.

Correct: Broken strands mean the cable has lost strength and can snap or bind, so it is replaced even though the parking brake still holds.

Incorrect: Adjusting the slack does not restore the broken strands, so the weakened cable can still fail.

Incorrect: A caliper piston seal leak wets the caliper and wheel, and no fluid was found at any wheel.

Correct: A leaking rear (secondary) seal in the master cylinder lets fluid run into the vacuum booster, where it cannot be seen; separating the master cylinder from the booster shows it.

Incorrect: Leaking wheel cylinder cups wet the linings inside the drum, and those were found dry.

Incorrect: The reservoir cap vent only lets air move as the level changes; glycol brake fluid does not evaporate through it fast enough to keep dropping.

Incorrect: A hose seeping under its bracket clip still leaves wet residue there, and no fluid was found at any hose.

Incorrect: Compression fittings are not approved for hydraulic brake lines; they can slip or leak under brake pressure.

Incorrect: Aluminum tubing is not approved for brake lines; it work-hardens and cracks under vibration and pressure.

Correct: ISO (bubble) flares seat only in ISO fittings, so the replacement line is steel with the same flare type as the original fittings.

Incorrect: Double (inverted) flares and ISO bubble flares have different seat shapes and are not interchangeable; a double flare in an ISO fitting will not seal.