Hard brake lever / pedal, difficult to press

Warning: braking is safety-critical — if you have any doubt, don't ride and get it checked by a professional. The lever (or pedal) is abnormally hard, almost blocked: even squeezing hard, braking doesn't respond like it used to, or travel is very short. This symptom points to the hydraulic circuit and its main components, almost all classified expert-only since they're at the heart of braking. Three main causes: a seized or clogged master cylinder (the piston struggles to slide and return, hard lever, weak braking), one or more seized caliper pistons (oxidation, dust, aged fluid: the piston no longer moves freely, hard lever and a brake that may drag and heat up), or very old, contaminated brake fluid (thickened, with deposits or water, that flows poorly). A key test: is the lever hard right from rest with little travel (master cylinder or seized caliper), or is travel normal but it becomes very hard to squeeze (contaminated fluid)? And does the disc get abnormally hot without braking (seized caliper)? Urgency threshold: a hard brake that doesn't brake properly, or a brake that stays applied and overheats/locks the wheel, is an immediate danger — don't ride, get it towed or repaired on the spot. Nearly all the work here (master cylinder, caliper, bleeding) is for a professional: you don't tinker with a hydraulic braking circuit without the skills. Better to spend 30 euros on a diagnosis than to be unable to stop or lock a wheel while riding.