Bike Maintenance: Tuning disc brakes

Disc brakes have been bringing mountain bikes to a stop for years. Recently, they’ve made the jump to ‘cross bikes. There have also been initial forays into getting the discs onto road bikes. If you don’t have this style of brake in your current stable of bikes, it’s likely to be there in the future. Here are some tips to help you with maintenance.

Disc or rotor rub can be a common problem with both mechanical and hydraulic disc brakes. To diagnose a rubbing rotor, first check its alignment. Much like truing a wheel, a rotor can be straightened if it’s slightly bent. If it’s really bent, it’s best to replace it as it will never stay true again.
With a mechanical brake, one that relies on a cable actuation, adjust the caliper so that it’s centred over the rotor and push the pads so they aren’t touching either side of the rotor. If the brake pads are not retracting after you release the brake lever, disconnect the cable and move the caliper lever arm manually to check for binding. If the caliper is functioning properly, the brake cable and housing, which can corrode and fill with debris, are often the problem. You’ll need to replace these parts.

With a hydraulic system, a straight rotor that consistently has rubbing issues is often caused by the caliper pistons not retracting fully into their bores. A good cleaning and lubing of the pistons can cure this issue most of the time. Sometimes, however, the piston seal degrades to a point at which the caliper stops functioning properly. Depending on your brand of brakes, some calipers can be rebuilt with new pistons and seals, while others have to be replaced.

If a piston clean doesn’t solve the rubbing issues, and before diving into a full caliper rebuild, there are a couple key areas to check. First, inspect the entire brake system for leakage. A leaking brake system that uses glycol-based brake fluid often leaves a trail of flaking paint on either the brake caliper, brake lever, or the bike frame. Mineral-oil systems are less corrosive and will not leave any paint blemishes. An easy way to determine whether your brakes are leaking is to pressurize the system by wrapping a cable tie around the brake lever while it’s depressed. Let it sit for a few hours and then check for brake fluid drips and/or a lack of firmness to the brake lever action.

If there are no signs of leakage in the system, a bleed is in order. Once the system has new fluid and is void of air, check to see if the pistons are finally retracting. If not, inspect the master cylinder. A faulty master cylinder will draw air into the system and a proper bleed will be impossible to achieve. The system will pump up with each squeeze of the brake lever and will feel spongy instead of firm despite the fresh bleed. Again, like a caliper rebuild, some systems can be rebuilt and others need replacement. Once you’ve determined if the master cylinder is functioning, the brake caliper is the only remaining culprit. As mentioned, it will either have to be rebuilt or replaced.

Brake calipers are very resilient and usually only degrade from a lack of maintenance or from a rubbing rotor. Heat generated through continued friction will cook the piston seals slowly and accelerate their degradation. The moral of the story: address brake issues, such as rotor rub, sooner rather then later as they can do more damage if left too long.