Brakes squeal / screech / judder

This is the most common and most benign cause of squealing while braking. Friction between the pads and the disc generates fine dust that builds up in the caliper, on the pad edges, and on the disc; combined with moisture and small high-frequency vibrations, it makes the brake "sing," especially at low speed or at the end of braking. Slight surface glazing on the pads can also whistle. In the vast majority of cases, this doesn't affect braking effectiveness and poses no danger: it's mostly annoying. The reassuring sign: bite stays good and there's no pulsation or dragging brake. The fix is an easy DIY job: clean the pads, the inside of the caliper, and the disc with brake cleaner (which evaporates without a greasy residue), which often eliminates the noise. For some stubborn squeals, a dab of anti-noise paste on the back of the pads (never on the friction surface) helps. Important: if the squeal persists after cleaning, or comes with reduced bite, a continuous metallic grinding, or pulsations, it's no longer simple dust — you need to check wear (indicator), the disc's condition, and whether the caliper slides freely. But on its own, with effective braking, this squeal isn't serious.

Many pads include an audible wear indicator: a small metal tab positioned at the minimum thickness spec. When the lining wears down to this level, the tab contacts the disc and produces a sharp, distinctive metallic grind while braking (and sometimes while riding). This is a deliberate signal from the manufacturer: it's time to replace the pads. At this stage, very little lining remains; if the noise is ignored, you soon reach the metal backing plate, which scores and destroys the disc (a much costlier repair), and bite drops. This is a grinding noise not to ignore, distinguished from benign dust squeal by its metallic character and by the drop in bite that often comes with it. Replacing the pads (by pair) is an intermediate, accessible job: push the pistons back, remove the retaining pin, fit new pads of the correct part number, pump the lever back up, then bed them in. On a vital component, rigor is required, and you check the disc's condition at the same time (pads left too long can have marked it). Confirming this cause: remove the pads to measure the remaining lining against the minimum spec.

A brake disc that's no longer perfectly flat (warped, from an impact or overheating) causes the thickness clamped by the pads to vary with each wheel revolution: this results in rhythmic pulsations in the lever or pedal while braking, and sometimes a noise. A disc too thin (below the minimum spec engraved on it) overheats too easily, deforms readily, and can crack. The distinguishing sign: pulsations follow the wheel's rhythm (faster as you go faster), unlike a constant squeal. Beyond the discomfort, a warped disc degrades braking and fatigues the pads and caliper. Precise diagnosis is done with a dial gauge (measuring warp) and a micrometer (thickness, compared to the minimum spec), and disc replacement is an intermediate job (removing the caliper, wheel, and disc), which doesn't open the hydraulic circuit but does affect a vital component — hence the rigor needed (disc bolts torqued with threadlocker, correct mounting direction, bedding-in). Pad replacement is often done at the same time as the disc. A compliant disc (dimensions, fixed or floating type) is essential. This cause overlaps with the "vibration while braking" symptom: it's the same phenomenon.

A seized caliper piston no longer retracts freely after braking: it keeps the pad in permanent contact with the disc. The result is a PERMANENT squeal or drag (even without applying the brake), overheating (a bluish disc, abnormal heat), rapid and uneven pad wear, and reduced performance/increased fuel consumption (the brake drags). Seizing comes from piston oxidation, accumulated pad dust, aged brake fluid, or deteriorated seals. The distinguishing sign: the noise and drag are there all the time, not just while braking, and the wheel in question turns stiffly (wheel lifted), with the disc getting abnormally hot. This is a cause to address without delay, since it destroys the disc and pads and can, in the extreme, lock the wheel. Since the caliper is part of the hydraulic braking circuit, restoring it (removal, cleaning and freeing the pistons, replacing the seals, greasing the slide pins, and above all bleeding the circuit) is classified expert-only: a mistake on this vital circuit can cause loss of braking. Leave it to a professional. In the meantime, if the brake is clearly dragging and the disc is getting very hot, don't ride: it's dangerous and destructive.