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Hydraulic Disk Brake Pad Replacement
and Caliper Centering

Disk brakes use two metal-backed pads held in place by magnets. There may also be a spring-loaded clip that keeps the pads in place. Pistons push the pads against a metal rotor. The piston may be activated by hydraulic fluid or a cable. If your brake is cable activated, click here.  This section is for hydraulic brakes without external pad-position dials, such as Hayes.  If your hydraulic brake depends on external knobs or hex-wrench adjusters to control the pad position, click here.

Proper function of the brake depends on (1) the rotor must be straight and smooth, (2) the caliper mechanism must be properly aligned with the rotor, (3) the pads must be positioned correctly, (4) there must be enough "pad" left, and (5) the lever mechanism must push the pads tightly against the rotor, with "lever" to spare.

Check rotor, pad thickness, and caliper position. Sight along the rotor (disk) to see how much brake pad you have left.
 See how much "daylight" you have between rotor and pad, and note the thickness of each pad. On some models, this can easily be done with the wheel in place. On Hayes, you need to remove the wheel and flip the bike over.

Spin the wheel. Check the rotor itself for warp and damage. Look for side-to-side wobble. See if the caliper opening is centered on the rotor.

If the rotor wobbles, is severely worn, or is otherwise damaged, click here.

Changing the brake pads:

To change the brake pads, remove the wheel. Grasp the handle of a brake pad. Move the handle towards the inside of the slot, and pull firmly. (On Avid and Hayes, there is a retaining clip that holds the pads. You need to overcome the tension on the clip.) The pad will slide out.

Insert each new pad. Pads are specific for inside versus outside (and top versus bottom). Push the pad back until you feel it click into place. Make sure the pads are seated correctly into the retaining clip, and positioned over the caliper piston. If a pad is inserted incorrectly, it won't work right.

This view has the bike turned over, looking at the underside of the caliper mechanism. The handles of the pads can be seen near the tip of the index finger.

Confirm that the pads are in proper position.

Aligning the caliper mechanism:

Loosen the two bolts that hold the main body of the brake onto the mounting yoke or fork. These bolts aim forward along the axis of the rotor, with one above and below the calipers. The holes for these bolts allow for some side-to-side "play" that positions the caliper mechanism. (Don't take the bolts out; just back off the tension in the bolt enough to allow the rotor to "shift" a bit on its mounting.) 

If the caliper is obviously crooked, you can nudge it a bit to center it on the rotor. Now insert a playing card along each side of the rotor. If you like a bit more handle motion before the brake engages, put two cards along the piston side of the caliper (the side that actually moves when the brake lever is pulled).
Now grasp the brake lever tightly to push the pads tight against the rotor. Have an assistant keep the brake lever tight.

While the pads are tight, tighten down both retaining bolts securely.

Remove the cards from the rotor. Spin the wheel to be sure the rotor is clear of the pads. If necessary, adjust again, altering the thickness and number of playing cards used to space the calipers.

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