Weak / spongy brakes

Brake pads are friction linings that clamp the disc to slow the wheel; they wear down naturally with mileage. When the lining gets too thin (below the minimum spec, often around 1.5 to 2 mm, or at the wear indicator — check your manual), bite drops noticeably: you need to squeeze harder for the same result, the lever stays firm but braking isn't what it used to be. Worse, pads worn down to the metal backing plate score and destroy the disc (a much more costly repair). This is a very common cause, easy to check visually through the caliper. Replacement, by pair (always both pads on a given caliper), is an intermediate job, accessible to a careful home mechanic since it doesn't open the hydraulic circuit: push the pistons back, remove the retaining pin, fit the new pads, pump the lever back up, then bed them in. Reputable brands (Brembo, Galfer, EBC) offer organic pads (softer, gentler on the disc) or sintered pads (better bite and durability). Important: on a vital component, be rigorous, and check the disc's condition at the same time. Uneven wear between the two pads also signals a dragging caliper piston, which needs attention.

Brake fluid hydraulically transmits the lever's force to the caliper pistons. It's hygroscopic: it gradually absorbs moisture from the air, which lowers its boiling point. Aged fluid (turned brown/dark) can then boil under the heat of sustained braking (a descent, repeated braking): the resulting vapor, being compressible, makes the lever spongy and braking fail — this is "vapor lock," a dangerous phenomenon. A low level (from pad wear pushing the pistons out, or from a leak) increases the risk of drawing in air. The fluid should be replaced periodically, often every 2 years, regardless of mileage (check your manual). Signs: a lever that softens especially when hot, dark fluid in the reservoir, braking that "fades" after several hard applications. Replacement requires a full bleed of the circuit with compliant fluid (usually DOT4), without leaving any air: a sensitive job classified expert-only, since an incomplete bleed leaves spongy, dangerous braking. Leave it to a professional if you're not perfectly equipped and confident. A level that drops on its own can also simply indicate worn pads (worth checking) or a leak to be located. Never neglect old fluid: it's an essential safety maintenance item.

Repair to be left to a professional.

Brake fluid is incompressible, which lets it fully transmit the lever's force to the calipers. But if air (compressible) gets into the circuit, squeezing the lever first compresses those bubbles instead of pushing the pistons: the lever goes soft, sinks, and braking sorely lacks bite. Air gets in during a poorly done bleed, when the reservoir level has dropped too low (badly worn pads pushing the pistons out), through a leaking hose or fitting, or after the fluid has boiled. This is one of the typical causes of a spongy lever that sinks. The only fix is a careful bleed of the circuit to purge all the air, after dealing with any leak or low level, using compliant fluid. This job is classified expert-only, since an incomplete bleed leaves dangerous braking: leave it to an equipped professional if you're not certain you can do it right. Since braking is vital, there's no compromise: as long as the lever sinks and braking is spongy, the bike shouldn't be ridden. This symptom is distinct from a firm lever with weak bite (which points to pads or glazing), because here the feel is clearly soft with abnormal sinking.

Repair to be left to a professional.

Effective friction between pad and disc requires clean, "live" surfaces. Two things degrade it. Glazing: under excessive heat (prolonged braking, a dragging pad), the lining surface hardens into a smooth, shiny layer that slides instead of biting. Contamination: grease, oil (a leaking fork seal dripping onto the disc, a weeping caliper, excess lubricant applied during assembly), or some greasy product reaches the friction area and drops the coefficient of grip. In both cases, the lever stays firm but braking is weak. This is an intermediate-level cause to address: thoroughly degrease the disc and pads with brake cleaner (never any greasy product on the friction surface), lightly sand pads that are only glazed to refresh them, or replace pads soaked in grease (the lining absorbs it and can't be recovered). If the contamination comes from a leak, you need to fix it at the source, or the problem will return. On a vital component, rigor is required, and a gentle bedding-in follows the repair. This cause is distinct from wear (where there's no lining left) and from hydraulics (where the lever is soft): here, lining is present and the lever is firm, but bite is missing.