Brakes: spotting wear without taking the wheel off

A disc brake consists of the disc, two pads, a caliper with one or more pistons, a line with a flexible hose, and the fluid inside it. Wear affects all of it, but not at the same rate and not in the same way.

What is almost always harmless: a brown film of rust on the disc after a damp night or a few days standing. It is gone after a few stops. Equally harmless is brake dust on the wheels — it says something about the pad compound, nothing about condition.

What is not harmless: a disc with deep grooves, a disc with a ring of rust at the outer or inner edge that stays there, and a caliper with an oily sheen on it. The ring of rust at the edge means the pads are no longer bearing there — either because they have become too narrow or because the disc has worn unevenly.

There is no universal millimetre figure for a brake disc, which is why none appears here. It is stamped or cast into every disc, usually on the hub or the outer edge, complete with its unit. That figure applies to that disc and is the only one that counts.

The same applies to the pad in a different form: the manufacturer states the permitted remaining friction material thickness, and it differs between front and rear axle and between cars. The backing plate does not count; measuring “still five millimetres” with the plate included is measuring optimistically.

At the technical inspection it is not the tester’s feel that decides but the measured performance on the brake tester plus the condition of the components. A car can pass on thin pads and fail on thick ones if a caliper is working unevenly.

With open-spoked wheels a lamp shows enough to answer the main questions. Three things to look for, none of which needs a jack:

Most cars have floating calipers: the piston sits only on the inboard side and presses the inner pad against the disc; the reaction force pulls the caliper inward on two guide pins and so presses the outer pad on. That works as long as the pins slide.

If the grease in the guides gums up, or a boot has split and let water in, the caliper seizes. Only the inner pad then works — and that is precisely the one you cannot see through the wheel. A car with a healthy-looking outer pad can be running on the backing plate inside.

Two pointers exist without removing anything: a ring of rust on the inner edge of the disc, visible with a mirror or a phone camera, and one wheel carrying noticeably more brake dust than the other. Either is a reason to take the wheel off after all before buying.

There are two designs. The mechanical one is a metal tab on the pad that touches the disc below a certain remaining thickness and produces a bright squeal — present while rolling, gone while braking. The electrical one is a wire loop inside the pad that gets severed and lights a lamp on the dash.

Both share the same limitation: they are not fitted to every pad. Often only one wheel per axle has one, sometimes only the front axle. A lamp that is not lit therefore means only that the monitored pad still has material. About the other three it says nothing.

On top of that, wear sensors are often forgotten or bridged when pads are changed. A lamp that has never lit is no proof — a look through the wheel is.

Brake fluid is hygroscopic: it draws water out of the air through hoses and seals. As the water content rises, the boiling point falls. In daily use you notice nothing — until the situation where the brakes genuinely get hot: a long descent, a loaded trailer, a hard stop from speed. Vapour then forms in the line, and vapour compresses. The pedal goes to the floor.

That is why the change is a time interval, not a distance interval, and why it falls due on a little-used car exactly as on a much-used one. Which period applies is in the manufacturer’s maintenance schedule. The water content is measured with a tester — and sensibly not in the reservoir but at the caliper, because that is where the water ends up and where it gets hottest.

A footnote that is often misread: a slowly falling fluid level in the reservoir is normal with disc brakes. The pistons extend further as the pads thin, and the extra volume comes from the reservoir. If the level falls quickly, it is a leak.

Three checks are enough, and all three need an empty, straight road. First: brake evenly from a moderate speed with your hands loose on the wheel. If the car pulls to one side, a caliper is not doing its share, or a flexible hose is acting as a valve inside — letting pressure in and not out, so that one brake drags.

Second: a firm stop from a higher speed. Pulsation through the pedal and the wheel points to uneven disc thickness. The common phrase “warped disc” usually misses it; more often it is pad material transferred unevenly onto the friction face, which shows up as a thickness variation under braking. For a buyer the distinction is academic — the disc gets machined or replaced either way.

Third: the parking brake on a slope. A car that creeps with the parking brake applied has either a cable brake needing adjustment or an electric parking brake with a seized actuator. The second is the expensive one.

Brakes are one of the few components that standing still harms. A disc left unturned for weeks rusts where the pad rests, and the pad can bake onto it. Pistons seize, guide pins gum up. A car with a strikingly low mileage for its age therefore needs a more thorough brake inspection, not a lighter one.

Regenerative braking creates the same effect. In an electric or hybrid car much of the deceleration is done by the drivetrain; the friction brakes only come in under heavy braking. The pads last a long time as a result, but the discs corrode, and calipers that rarely work are more likely to seize. Some manufacturers answer that with drum brakes on the rear axle, which are less exposed to corrosion.

For viewing an electric car that means pad thickness is rarely the issue. The state of the disc faces and the calipers is.

Pads and discs are wear items with known prices; a quote takes one phone call and works as evidence. A seized caliper is a step above that. An electric parking brake with a failed actuator is a step above again, and on an older car it can tip the whole calculation.

And one line that is not negotiable: a metallic grinding under braking. That is the backing plate running on the disc. A car in that state does not get test driven and does not get driven home; it gets put right where it stands.