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Thread: Hydro Questions

  1. #1
    I Tried Them ALL... Moderator
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    Hydro Questions

    Why do hydraulic brakes need a card to go between the pads, when removing the wheel? are the pads always in contact with the rotor? I'm still using Avid mechanical discs, and cannot imagine riding on my brakes, all the time.

  2. #2
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    Quote Originally Posted by Zachariah
    Why do hydraulic brakes need a card to go between the pads, when removing the wheel?
    You don't need to put a spacer in between hydraulic brake pads when the rotor is removed. Just be careful to not squeeze the brake lever when the rotor is not in place. If you do, the pads will be squeezed together and you will need to pry them apart before the rotor can be put back in.
    are the pads always in contact with the rotor?
    No, the pads are not always in contact with the rotor.

  3. #3
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    Oh, the pads don't retract back out(seperate) when the lever is released? That is not right....

  4. #4
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    To add to Kevin's post, I'll give you the long-winded reason. First of all, cable operated brakes use a spring to return the pad(s) to their original position prior to the lever being pulled. This is why is doesn't matter about pulling the lever when the rotor isn't between the pads.

    In a hydraulic system, the position of the pads is determined by the postion of the pistons. The piston seals, which are seated in the caliper body, are responsible for restricting the movement of the pistons. The square-edge design of the seals will ensure that the piston is always returned to a specific position relative to where it stopped (ie when the pad touched the rotor) when the brake lever was applied. Basically, however far the piston extends when the lever is pulled it will always only return by a set distance, for example 1mm. As the pads wear, the pistons have to extend further from the caliper to press the pads against the rotor, but they will always only return by the 1mm that the seal has determined. The extra space created in the hydraulic line from the pistons extending further is filled with fluid from the reservoir.

    This is why hydraulic brakes are self adjusting, and also why we need to take care not to pull the lever when the rotor is out of the caliper. As Kevin said, it's no big deal if you do close the pads, they can (usually be) easily opened up again with the assistance of a large, flat-head screwdriver.
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  5. #5
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    Quote Originally Posted by SteveUK
    As Kevin said, it's no big deal if you do close the pads, they can (usually be) easily opened up again with the assistance of a large, flat-head screwdriver.
    Or a pocket knife blade. Use it like a wedge and push it between the pads. That way you don't mar the pads like what can happen when using a screwdriver.
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