Thursday, 2 October 2014

How to reset mountain bike disc brakes - calliper pistons

When the brake pads are replaced or if accidentally the brake lever is pressed without a rotor or spacer between the pads, you will have to reset the brake calliper pistons. Basically resetting your brake pistons is just only pushing your calliper pistons back to their original position. It is an easy task, and will ease the new brake pads installation.

Shimano M615 brake with the pads fully retracted
Shimano M615 brake calliper with fully retracted pads as it comes from the factory

  1. If you are going to change or upgrade your mountain bike brake pads the easiest way to reset your pistons is without removing the worn brake pads. Remove the wheel and just put a large flat blade screwdriver between the pads and twist pushing the pads apart until the calliper pistons are retracted to its original position. You will get enough room to clear the new brake pads and the brake rotor.

    Tire lever to retract the mountain bike brake pistons
    Push the pads apart until they are fully retracted to the original position

    If you want to use this method but you don’t need replacing the brake pads and want to avoid scratching your current brake pads, you can use any other softer tool (tire lever for example), or just wrap the screwdriver blade in electrical tape and follow the same twisting procedure. 
    If the brake pistons don’t move back enough to clear the brake rotor, or there are very hard to retract is often because there is too much brake fluid in the system causing the lock. In this case you will have to remove any excess brake fluid from the system, unscrewing a bit the top bleeder at the master cylinder or removing the fluid reservoir top cap. Always try first with the top bleeder or reservoir top cap to avoid air entering the system. Catch the excess brake fluid with some paper towel and be careful and cover your brake pads to avoid brake fluid splash ruining them. Tighten the bleeder or reinstall the reservoir top cap and you are done.

  2.  If you have removed the brake pads, you can retract the calliper pistons with a tire brake lever or similar “soft” tool to avoid scratching the surface of your calliper pistons. Press both pistons until they are fully retracted into the calliper body.

  3. Now install the new pads, the wheel and “set” the brake pistons to the correct position by pressing the brake lever hard 2-3 times. After this setting you should have full power, but if you have removed some fluid from the brake system and feel your brake spongy, you should bleed your brake system following the Service Instructions of your brake system manufacturer. Search the links below for detailed information from the main mountain bike brake manufacturers.

  4. Shimano M615 brake calliper after piston setting
    Shimano M615 brake calliper after piston setting

Bleeding TRP brakes

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