Brake pads wear down slowly, and drivers get used to the car stopping a little worse every month. That is why wear is usually noticed by ear rather than by feel, and often only after the disc has been damaged. But noise can mislead: not every squeal means wear, and on some cars there will be no warning squeal at all. Here is how to tell the difference, what you can genuinely check yourself, and when the car should not be driven anywhere.
Noise: which squeal means wear, and which does not
Most pads carry a mechanical wear indicator — a thin metal tab. When about 2-3 mm of friction material is left, the tab starts touching the disc and produces a high-pitched squeal while the car is rolling: it is easiest to hear at low speed with the window down, and it often stops the moment you press the pedal. Some cars use an electrical sensor with a dashboard light instead, but the sensor is normally fitted to only one or two wheels out of four, diagonally — so a light that stays off does not prove the other pads are fine. The sensor is single-use and is replaced together with the pads, and some original-equipment pads have no indicator at all: there will be no warning, only inspection.
Squealing without wear is common. In the morning and after a car wash, pads squeal against a film of rust on the disc and go quiet after a few stops. A persistent squeal that appears specifically when you brake usually comes from the pad vibrating in its carrier, because anti-squeal shims were left out or the pad contact points in the carrier were not greased at the last replacement. Cheap pads with a hard compound squeal on their own dust. The distinction is simple: the wear indicator is heard while coasting and fades under the pedal, while a vibration squeal does the opposite — it shows up at the moment of braking.
A rough metal-on-metal grinding sound is no longer a warning: the friction material is gone, the steel backing plate is cutting into the disc, the coefficient of friction collapses, so stopping distance really does grow and a pair of new discs joins the bill. Up to that point a thin but intact pad barely lengthens braking — the limit is set by the tyre anyway. What is lost is thermal capacity: on a long descent or after several stops in a row from motorway speed, worn pads fade earlier, the pedal goes hard and deceleration turns soft. With grinding you head to the workshop straight away, without long trips.
Pedal feel, fluid level and vibration
Worn pads on their own barely change pedal travel, because the caliper piston moves out as the material wears. What does change is the fluid level in the reservoir: the fluid fills the caliper cylinders behind the extended pistons. If the level sits at the minimum mark and there are no leaks or damp hoses anywhere, that is an indirect sign the pads are thin. And in that situation you must not top up to MAX: when the pads are replaced the pistons are pushed back in, the reservoir overflows, and fluid ends up on the paint and on the ABS unit. And if the level has already dropped below the MIN mark, it is not something to leave for a convenient day: the warning light comes on and air can be drawn into the circuit, so the brakes get checked straight away.
A soft pedal is a separate fault and has nothing to do with pad thickness. If the pedal travels slightly further than usual but still holds firm, book a brake inspection within the next few days. If the pedal sinks to the floor, or slowly drops under your foot while you wait at a light, the car does not drive anywhere under its own power — not to the workshop, not "just around the corner": call a tow truck. The usual culprit is the master cylinder (classic pattern: the pedal sinks slowly and there are no leaks), less often air and aged fluid or a swollen brake hose. A seized caliper almost never produces a soft pedal: it shows up as pulling to one side (on a car with ABS and ESP the pull is masked almost to the limit), one overheating wheel and unevenly worn pads.
Pulsing through the pedal when braking from speed means the disc is not the same thickness all the way round. Genuine heat warping is rare; in most cases pad material transfers onto the disc unevenly and builds up in patches (disc thickness variation), helped along by runout — rust and dirt on the hub mounting face under the disc, and less often a worn wheel bearing.
So the verdict comes from measurement, not from feel. The minimum thickness is stamped on the disc itself — on its hat or around the outer edge, not on the hub — and it has to be measured with a micrometer at 6-8 points around the disc: a vernier caliper runs into the lip at the rim and gives a false figure. If there is margin left, the disc is skimmed, ideally on the car; if there is none, it is replaced, and in pairs per axle, just like pads. Before a new disc goes on, the hub mounting face is cleaned back to bare metal and runout is checked with a dial indicator (typically no more than 0.05 mm), and the first 100-200 km are driven without hard stops from high speed — otherwise the deposits and the pulsing come straight back.
What you can check yourself — and where that stops
- Friction material thickness. A new front pad has roughly 10-12 mm, a rear one more often 8-10 mm, and on some cars 7-9 mm. Plan the replacement at around 3 mm and treat 2 mm as the limit. Important: those figures refer to the friction material without the steel backing plate, while some manufacturers quote the limit as total thickness including the backing — check the manual, or you will be out by a factor of two.
- Where to look. Through the wheel spokes you can see the outer pad, and only roughly. On most cars the inner pad cannot be seen through the spokes even with a torch — the wheel has to come off (on some models it is visible from above, through the arch). And it is the inner pad that wears faster: it sits on the piston side, and sticking slide pins work against it too. So a look through the spokes is a rough estimate that consistently makes the pads look better than they are; the verdict is made with the wheel off.
- The difference between the inner and outer pad on the same caliper. If one is clearly thinner, on a floating caliper a slide pin or the piston is sticking; on a fixed multi-piston caliper it is the piston (there are no slide pins there). Fitting pads alone changes nothing in that case — the caliper has to come apart and be serviced, otherwise the new pad wears just as unevenly.
Two situations where all of the above stops applying. The first is an electronic parking brake (EPB) on the rear axle: the piston cannot simply be pushed back, it needs the service or retraction mode via a scan tool or the manufacturer's procedure, otherwise the caliper motor is destroyed. On cars from roughly 2015 onwards that is the main reason rear pads are not a driveway job. The second is hybrids and EVs: regenerative braking means the pads can last well over 100,000 km, so they are replaced because of corrosion rather than wear — a rusted disc, a pad stuck to it, seized slide pins. Measuring thickness on such a car tells you almost nothing.
What to do now: with the car cold, check the brake fluid level and look at the outer pads through the wheels. If you hear a constant squeal while rolling, the wear light is on, or about 3 mm is left, book a pad replacement — pads are changed in pairs, axle by axle. If there is grinding, pedal pulsing, pulling to one side or one wheel running hotter, you need a brake system inspection with the wheels off, because pads alone will not be the whole job. A set of pads costs considerably less than a pair of discs, and a seized caliper adds a third item to the same bill. And if the pedal sinks to the floor, do not drive at all — call a tow truck.