Disc brakes: maintenance, bleeding and pads

Two complaints bring disc brakes back to the workbench nine times out of ten: a lever that goes soft and spongy, or pads that squeal. They sound like the same problem to a customer — "the brake isn't like it used to be" — but they're mechanically unrelated, and telling them apart before you crack open the caliper saves everyone time.

Reading the symptom before you open the circuit

A lever that pulls smoothly to the bar with a soft, progressive feel almost always means air in the line or a leaking seal — that calls for a bleed, not an adjustment. A lever that feels firm but the brake barely bites, or squeals under braking, points to the pads instead: worn past their minimum thickness, glazed from heat, or contaminated by chain lube, grease, or overspray from a polish that landed where it shouldn't. A sharp, metallic screech is almost always a pad worn down to its backing plate grinding on the rotor — no bleed will fix that, only replacement, and fast, before the rotor gets scored.

A quick bench test: with the wheel out, pump the lever with nothing in the caliper. If the pistons extend slowly or unevenly, or one lags behind the other, the fault is in the hydraulic circuit. If the pistons move cleanly but the pads are under a millimetre of friction material, the diagnosis is already made.

Bleeding: DOT and mineral oil don't mix

Before you even open the bleed kit, confirm the correct fluid. DOT systems (SRAM, some Shimano road lines, most road and gravel brakes) run DOT 4 or DOT 5.1 fluid, while mineral-oil systems (most Shimano MTB groups, Magura, some Tektro) run a proprietary mineral oil. Mixing them up isn't a minor slip: DOT fluid in a mineral circuit swells the seals and destroys them within a handful of rides, and mineral oil in a DOT circuit fails to lubricate correctly and corrodes parts built for DOT. The fluid type is stamped on the lever or caliper body, and it's worth checking every single time — especially on a bike that isn't new to the shop or has been worked on elsewhere.

The bleed procedure itself follows the same logic across nearly every system: a syringe (or the dedicated funnel) goes on the lever, another on the caliper, and fresh fluid gets pushed from the bottom up, forcing bubbles out while keeping the circuit full so air doesn't get pulled back in. Three things separate a bleed that holds from one you're redoing in two days. First, orient the bike the way the manufacturer specifies — usually lever level or tipped slightly up, so bubbles rise toward the bleed port. Second, always flush the old fluid completely rather than topping it up. Third, before sealing everything back up, tap gently along the hose run with a non-metallic tool to knock loose the microbubbles clinging to the walls. A properly bled brake gives a lever that stops at the same point, pull after pull; if the lever creeps toward the bar under repeated braking, either air is still in the system or there's an upstream leak worth chasing before you close it up.

Pads: material, bedding-in and the ECE-R90 mark

The choice between organic (resin) and sintered (metallic) pads matters more than it looks. Organic pads bite sooner and run quieter but wear faster and struggle with the sustained heat of long descents; sintered pads last longer and handle wet or muddy conditions better, but they're noisier and harder on the rotor, which needs to match — many sintered-compatible rotors don't pair well with organic pads, and vice versa. Every pad swap needs a proper bedding-in: roughly ten progressive stops from moderate speed down to a near-halt, never locking the wheel, to lay down an even layer of friction material on the rotor. Skipping this step is the single most common reason new pads squeal for weeks.

One thing worth explaining to customers, especially on eBikes where braking loads run higher: the ECE-R90 mark found on some pads and rotors certifies friction materials to the same homologation standard used for motor-vehicle brakes. On a pedal-assist bike, where the masses and speeds involved sit closer to a moped than a traditional bicycle, that's not a marketing footnote — it's a reasonable criterion for anyone covering serious mileage or riding hilly routes.

Brake work is one of the highest-trust jobs a shop does — customers who've had a brake fixed properly tend to talk about it. If disc brakes are part of your daily bench work, take a look at how CrankPal for workshops ties together service history, parts and customer communication.