Ask a Mechanic: Hydraulic Disc Brake Maintenance

Bill wants to know, what kind of maintenance should I be doing for my hydraulic disc brakes?

Maintaining your brakes is one of the most important things you can do to ensure a fun, safe ride when you jump on your bike. Any time you’re working with disc brakes, it’s important to keep things clean and free of contaminants.  Contaminating the pads or rotor will compromise braking performance, possibly to an unsafe extent. With that said, here are some tips for checking your brakes and procedures for combating some common issues that may arise.

Under regular riding conditions, disc brake noise can be caused by a number of things, but it’s usually a sign of old pads, a misaligned caliper, or a bent rotor.

First, remove the wheel and visually inspect the pads in the caliper.  If it looks like the pad material is wearing down and starting to get close to the backplate, it’s a good idea to pull them for a closer look.  If you determine the pads are worn and it’s time to replace them, there’s more to it than just throwing in a new set of pads.

As pads wear, the pistons in the caliper extend to accommodate for the thinner pad, so before installing new pads, the pistons need to be reset and pushed back into the caliper.  I usually end up using the handle of a cone wrench because they fit in the caliper nicely, and their rubber coating helps to protect the piston and caliper from damage.  Don’t use something sharp or too hard because you could damage the piston and ruin the brake.  It’s also important to press the piston in the center and have it retract as straight as possible.  You will likely have to go back and forth from side to side a few times before both pistons are fully retracted.

Once the pistons are reset and you have the new pads in, you’ll need to check and make sure that everything is still tight and secure.  Start by making sure the rotor bolts are torqued down to the manufacturers spec.  On a 6 bolt rotor, torque the bolts in a star pattern.  Centerlock rotors have only one lockring, so once it’s torqued to spec it will be good to go.

Once the rotor bolts or lock ring are torqued, put the wheel back in and check to see if the caliper is properly aligned with the rotor.  Ideally you want the caliper to be centered around the rotor with equal space on either side.  The easiest way to do this is to loosen both caliper bolts just enough so the caliper moves freely side to side.  Once the caliper is loose, squeeze the brake lever for the caliper you are aligning and hold it down while you re-tighten the brake bolts.  Release the brake lever and check to see that the caliper is aligned with even space on both sides of the rotor.

With some internally routed brake hoses this method may not work because the hose itself can put pressure on the caliper, moving it one way or the other.  Remedy this by adjusting the angle of the banjo nut that the hose is attached to by loosening the bolt just enough to get it to move. Then rotate it to an angle that doesn’t put added stress on the line and smoothes out the bend.  Adjustable banjo nuts are more common on mountain brakes, so if there isn’t an adjustable banjo you may have to use a little trial and error to get the caliper properly aligned.  Loosening the bolts and aligning the caliper by hand can take several tries but is worth the effort once it’s done.

The most common cause of disc brake headaches would have to be a bent rotor.  Bent rotors are quite common, especially on mountain bikes where rotors are typically much larger and exposed to damage from trail obstacles.  Sometimes even brand new rotors come out of the box with enough of a bend to cause a rub.

Although bent rotors are more common on the mountain it can happen on the road too, but the solution is the same for both, you’ll have to bend it back.  While it’s easier to fix a bent rotor with the wheel in a truing stand, you can also leave the wheel in your frame. Dedicated rotor-truing tools make the job easier and more precise, but a crescent wrench works in a pinch; just scrub the jaws well so nothing will contaminate the rotor.

Rotate the wheel and visually inspect the rotor going through the caliper, looking for points of contact. It can often be very hard to see exactly where the rotor is hitting the pads, so getting an ear close to the rotor and listening for the rub works well, especially for smaller bends.  Once the bend in the rotor is identified, move that spot out of the caliper and bend it slightly in the opposite direction. Work in small increments to avoid over-bending the rotor and making it rub on the other side.  If you’re using a truing stand attach a zip tie to the arm of the truing stand on the disc side with the tail pointed towards the rotor, and trim the zip tie to almost touch the rotor. Spin the wheel and watch to see where the rotor either hits the zip tie or moves away from it, and bend the rotor back in the opposite direction, again working in small increments.  Once the rotor is straight on the truing stand move it to the bike and double check to make sure it’s not rubbing.  Fixing a bent rotor can be quite time consuming and frustrating at times. However, a rub-free brake is worth the effort, so don’t get to frustrated if it takes a little longer than expected.  Once you’re done you should have a brake and rotor combo that is working to its full potential and no longer gives you and your friends a headache while out on the road or trail.