ASE A5 disc brakes, measured right

Correct: Rust scale under the rotor hat cocks it on the hub, so the mounting faces are cleaned and the runout rechecked before anything is machined or replaced; indexing comes next if it is still high.

Incorrect: Over-torquing the lug nuts distorts the rotor hat and can add runout; it does not seat the rotor through rust.

Incorrect: The rotor was never checked on a clean flange, so there is no evidence it is defective.

Incorrect: On-car machining can correct remaining hub runout, but cutting a new rotor to make up for rust scale removes metal for a problem that cleaning fixes.

Incorrect: Bearing play should be checked, but replacing the hub bearing before cleaning off obvious scale is premature.

Incorrect: Thickness variation pulses the caliper only during braking, so it cannot cause a shake while coasting.

Incorrect: A warped front rotor would make the shake appear or worsen when the pads clamp it, but this vibration does not change with braking.

Incorrect: Seized slide pins cause uneven pad wear or drag, not a speed-related shake that is the same with or without braking.

Correct: A shake that is there while coasting and does not change when the brakes are applied comes from the rotating wheel and tire, not from the brakes.

Incorrect: Glazed pads reduce stopping power or squeal; they do not create a highway vibration.

Incorrect: Machining it to 24.0 mm takes the rotor to its discard limit, the point where it is scrapped; a cut is never taken down to it.

Incorrect: Machining the rotor as planned ends at 24.4 mm, below the minimum the manufacturer allows after a cut.

Incorrect: Machining it and then measuring only confirms afterward that the rotor is under 24.5 mm; the math shows that before any metal is removed.

Incorrect: Indexing changes where the rotor sits on the hub to reduce runout; it cannot clean up the friction surface.

Correct: Replacement is required: removing 0.4 mm leaves 24.4 mm, under the 24.5 mm minimum allowed after a cut, so the rotor cannot be cleaned up within spec.

Incorrect: A proportioning valve sets front-to-rear pressure and affects both pads on an axle equally.

Correct: With one slide pin seized, the caliper cocks as it applies, so one pad is pressed at an angle and wears tapered while the other barely works.

Incorrect: Low fluid lowers the pedal or lights the warning lamp; it does not change how one pad meets the rotor.

Incorrect: A torn caliper piston boot lets dirt and water reach the piston and leads to sticking later, but by itself it does not wear one pad at an angle.

Incorrect: Worn shims make noise; they sit behind the pad and do not tip it at an angle.

Incorrect: A dial indicator reads side-to-side wobble (lateral runout) as the rotor turns; it cannot measure thickness between the two faces.

Incorrect: A micrometer measures thickness at one point, so it is the wrong tool for lateral runout, the face wobble seen as the rotor turns.

Correct: A micrometer is the tool for thickness variation: the variation is the difference between micrometer readings taken around the rotor.

Incorrect: Feeler gauges measure gaps, not the small face wobble of lateral runout.

Incorrect: A straightedge checks flatness across a surface; it does not measure thickness differences around the rotor.

Correct: Pads with different friction ratings make one side grip harder than the other, so braking force is unequal and the vehicle pulls.

Incorrect: The weaker pad limits its side, so mixing does not add stopping power.

Incorrect: Reduced noise is not the effect: compounds with different friction ratings vibrate differently, so mixing them does not quiet the brakes.

Incorrect: Mixing pad compounds does nothing to improve how heat leaves the rotor.

Incorrect: Pads soaked by a leak are replaced whatever their lining thickness, so thickness is not the check that matters.

Incorrect: The rotor is cleaned of fluid, but its surface condition does not explain the leak.

Correct: A leaking caliper almost always leaks at the piston seal, and the bore behind it is checked for pitting before deciding to rebuild or replace.

Incorrect: Fluid color says something about fluid age, not where a caliper is leaking.

Incorrect: New wheel bearings do not touch the pads, so they cannot cause a brake squeal on application.

Incorrect: New brake fluid has no effect on how the pads vibrate against the rotor.

Incorrect: Cleaning the caliper and its slides is part of a quiet pad job, not a cause of noise.

Correct: Shims damp the high-frequency vibration between pad and caliper, and leaving them out is a common cause of squeal after a pad job.

Incorrect: Low fluid lowers the pedal or lights the warning lamp; it does not create a vibration.

Correct: Uneven rotor thickness pushes the caliper pistons in and out each turn, and the pulse is most noticeable under light pedal pressure.

Incorrect: An unbalanced wheel shakes whether or not the brakes are applied, so it would not appear only during braking.

Incorrect: A soft compound changes bite and dust, but a uniform pad cannot create a once-per-turn pulse.

Incorrect: Pedal free play is set at the pedal and master cylinder; one sticking caliper does not change it.

Incorrect: The booster supplies assist to all wheels and cannot single out a dragging caliper.

Incorrect: Fluid level does not change when a piston sticks, so it says nothing about one wheel dragging.

Correct: A piston that does not fully release keeps the pad rubbing, so that rotor runs much hotter than the one on the other side, which an infrared thermometer shows quickly.

Incorrect: Piston size is a design dimension that does not change in service.

Correct: A torn dust boot lets water and dirt reach the piston, which corrodes and sticks, so it is checked every time the piston is pushed back.

Incorrect: Piston material (steel or phenolic) is fixed by the caliper design and is not a service check.

Incorrect: Piston color is cosmetic; the boot and the piston surface are what show damage.

Incorrect: Worn pads add travel and noise, but fade is a temporary loss of friction from heat.

Incorrect: Low fluid gives a low pedal or a warning lamp, a steady problem that does not come and go with heat.

Correct: Excessive heat in the pads and rotors lowers the friction between them, so stopping power fades until they cool.

Incorrect: A failing master cylinder gives a sinking pedal at any temperature, not fade that recovers after cooling.

Incorrect: Technician A is wrong: brake cleaner removes dust and oil but does not take off the hard glazed layer.

Correct: Technician B is right: glazing hardens the friction surface and the pad keeps squealing, so replacing the pads (and checking the rotors) is the reliable fix.

Incorrect: Both cannot be right, since cleaning and reusing the pads leaves the glaze that causes the squeal.

Incorrect: 'Neither' is wrong because replacing the glazed pads, as Technician B says, does correct the squeal.

Incorrect: Air makes the pedal spongy; it cannot make the pads click or wear unevenly.

Incorrect: Loose bearings can knock the pistons back and lower the pedal, but the click on apply and inner-pad wear point at the pad hardware.

Incorrect: A restricted master cylinder port affects apply or release at every wheel, not one pad's movement in its bracket.

Correct: Without good abutment clips the pad shifts in the bracket as the brakes apply, which clicks, and it can cock and wear unevenly.

Incorrect: The rotor measured uniform, and a rotor problem would pulse rather than click once at apply.

Correct: The caliper slides on its pins: piston force pushes the inboard pad, and the reaction pulls the caliper body and the outboard pad in against the rotor.

Incorrect: A spring clip holds the pad and limits rattle against the rotor, but it does not supply clamping force.

Incorrect: Pistons on both sides belong to a fixed (opposed-piston) caliper, not a single-piston floating one.

Incorrect: The rotor is not meant to flex; it stays put while the caliper moves around it.