Vibration / pulsing in the lever when braking

A warped brake disc is no longer perfectly flat: it undulates slightly sideways. With every wheel rotation, the pads clamp it sometimes harder, sometimes softer, which makes the lever or pedal pulse at the rhythm of rotation — hence the vibration felt under braking, which speeds up with speed. Warping comes from an impact, overheating (very prolonged braking on a descent, a dragging pad), uneven tightening of the disc bolts, or sometimes fitting new pads onto a disc that was already slightly deformed. This is the most frequent cause of braking pulsation. Precise diagnosis is done with a dial indicator (measuring lateral runout with the wheel turning, compared to manufacturer tolerance); while at it, thickness is also measured (a warped disc is often worn too). Replacing the disc is an intermediate-level job (removing the caliper + wheel + disc) that doesn't open the hydraulic circuit but does involve a vital component: care is needed (correct disc spec, bolts to torque with thread-locker, correct orientation, bedding in the pads). Although light pulsing still allows careful riding, a warped disc degrades braking and wears out the pads and caliper faster: get it replaced. This cause overlaps with the "vibrating brakes" symptom.

When the two pads of the same caliper don't wear the same way — one noticeably more than the other, or at an angle (thinner on one edge) — contact with the disc becomes uneven and can generate pulsing and vibration under braking. The underlying cause is almost always a caliper piston that's dragging or sliding poorly: it keeps one pad in more contact than the other, or distributes pressure unevenly, creating this asymmetric wear. On an opposed-piston caliper, it's a piston not retracting properly; on a floating caliper, it's seized sliding pins. So this is as much a symptom as a cause: it reveals a caliper problem to address. Diagnosis is accessible: remove the pads and compare their thickness and shape. Replacing the pads (as a pair) is intermediate-level, but you must also address the cause of the uneven wear (a seized caliper / sliding pins to clean and grease), otherwise the new pads will wear the same crooked way. Since servicing the caliper touches the braking circuit, the seized-caliper part is best left to a professional. On this vital component, also check the disc's condition. Don't just replace the pads without understanding why they were wearing at an angle.

Every brake disc has a minimum thickness stamped on it (a "MIN TH" mark or similar). Below this spec, there's not enough material left: the disc heats up much more (less mass to absorb and dissipate heat), warps easily when hot, and can even crack. The result is pulsing and unstable braking, especially after a few hard stops that have heated it up. An undersized disc must be replaced: this is mandatory for safety, not optional. This cause is often confused with a warped disc (a worn disc warps easily); diagnosis involves measuring thickness on the friction track with a micrometer/caliper and comparing it to the stamped minimum spec. Replacement is an intermediate-level job (same steps as for a warped disc: removal, correct disc spec, bolts to torque with thread-locker, bedding in the pads), which doesn't open the hydraulic circuit but remains a vital component — care is needed. Pads are often replaced at the same time. Important: unlike some car discs, a motorcycle brake disc that's too worn is never machined/resurfaced: below the minimum spec, replacement is mandatory. Riding with an undersized disc is dangerous (risk of cracking under thermal stress).

Warning: this cause matters for safety. Vibration under braking doesn't always come from the brake itself: a poorly tightened wheel axle or a wheel bearing with play lets the wheel "dance" slightly, resulting in vibration and instability, made worse under braking (when forces are at their maximum). A wheel bearing wears with mileage, water/dust ingress (a dead seal, high-pressure washing), or poor tightening; an insufficiently tightened wheel axle (after a repair, for instance) creates play directly. The tell: with the wheel raised, grip the tire at the top and bottom and check for lateral play, and spin the wheel to detect a rough spot or a rumble (bearing). This is a cause to check alongside the discs, since it affects wheel integrity: a failing wheel bearing or a loose axle are dangerous. Torquing the axle is accessible, but replacing a wheel bearing needs specialized tools (a puller, a press) and is done in pairs: leave it to a professional, given the safety stakes. As long as wheel play is present, don't ride: this is a priority safety defect.