Hard brake lever / pedal, difficult to press

The master cylinder is the part that, driven by the lever or pedal, pressurizes the brake fluid to send it to the calipers via the hoses. Its piston slides in a bore and returns under spring force, passing through a small compensating port connected to the reservoir. Over time, the internal seals age and swell (especially in contact with moisture-contaminated fluid), the bore oxidizes or gets dirty, and the compensating port can clog: the piston slides poorly, the lever becomes hard and travel is reduced, braking is weak. This is one of the typical causes of a lever that's hard right from rest. Fixing it (a rebuild kit with seals and spring, or full master cylinder replacement) then requires a full bleed of the circuit. Since this is the heart of the hydraulic braking system, this job is classified expert-only: a mistake (seal fitted wrong, port not restored, incomplete bleed) can cause a stuck brake or loss of braking. Must be left to a professional, with compliant parts and fluid and a validated test ride. This is a non-negotiable safety component: don't get back on the road until the lever has regained normal travel and firmness and clean braking.

Repair to be left to a professional.

The caliper contains one or more pistons that, pushed by hydraulic pressure, clamp the pads against the disc, then retract slightly at rest thanks to the elasticity of the seals. When a piston seizes — from oxidation (moisture, road salt), accumulated pad dust, aged brake fluid, or deteriorated seals — it no longer slides freely: the lever becomes hard, braking is weak, and often the brake drags (the pad stays in contact), which makes the disc get abnormally hot (it can turn bluish) and wears the pads quickly. This is a common cause, especially on a bike used little or exposed to moisture. The tell: a disc that's abnormally hot after a ride without heavy braking, and a wheel that turns stiffly (wheel lifted). Restoring the caliper (removal, freeing and cleaning the pistons, replacing the seals with a kit, greasing the slide pins, then bleeding the circuit) touches the hydraulic braking circuit: it's classified expert-only, since a mistake can cause loss of braking. Leave it to a professional. As long as the brake drags and the disc gets very hot, don't ride: it's dangerous (risk of locking up when hot) and destructive for the disc and pads.

Repair to be left to a professional.

Brake fluid ages: being hygroscopic, it absorbs moisture from the air, picks up deposits and oxides from internal corrosion in the circuit, and darkens (turning brown, almost black). Badly degraded, it can thicken, carry particles that clog the calibrated passages (notably the master cylinder's compensating port), and hinder the pistons' free movement: braking becomes hard, the control less progressive. This is a cause in its own right of a hard brake, often associated with a master cylinder or caliper starting to seize (the contaminated fluid is the root cause of their seizing). The tell: lever travel roughly normal but very hard to squeeze, and above all fluid visibly dark in the reservoir. The remedy is a full bleed of the circuit with fresh compliant fluid (usually DOT4, or per the manufacturer; never mix silicone DOT5 with DOT3/4/5.1), a sensitive job classified expert-only since an incomplete bleed leaves dangerous braking. Leave it to a professional if you're not perfectly equipped. Prevention: replace the fluid periodically (often every 2 years, check your manual), regardless of mileage. This is essential safety maintenance, too often neglected.

Repair to be left to a professional.