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dragging brakes question

Discussion in 'XJ Technical Chat' started by joejr2, Jul 25, 2023.

  1. joejr2

    joejr2 Active Member Premium Member

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    Brakes are not locked. the wheel can be pushed around. my question refers to a you tube presenter who demonstrated a way to tell if the dragging problem is in the master cylinder or the calipers. He said that if
    one taps on the calipers and the wheel turns easier, the pistons are not retracting properly and to look for the problem in the calipers. any opinions ? I tried that and the front wheel loosened up a lot but still dragged a little bit.
     
  2. Roast644

    Roast644 Well-Known Member

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    Dragging calipers are most often because the guide pins need lubed. The pins allow the caliper to move sideways with respect to the mounting point. The small bellows shaped boots seal them, but will allow lube to leave and grit to enter if torn.

    There's several things that could cause the caliper piston to not retract, also. But this seems to be a much more infrequent issue than guide pins.
     
  3. joejr2

    joejr2 Active Member Premium Member

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    With all the brake parts either rebuilt or replacced; calipers rebuilt w/new Pistons, seals, lube pins etc. master cylinder rebuilt w/chacal's kit (return hole clean).
    New SS brake hoses. Everything clean everything rebuilt. Bled lines, got good resitance on the lever. Blocked up the wheel spun it tested the brakes, wheel stopped
    instantly but there was rubbing. I tapped on both calipers and the wheel spun freely again. I disassembled the whole system again, looked at every thing, tested the calipers with air and pushed the pistons
    back in by hand, they moved back in easily. Reassembled everything, bled the lines, got lever resistsnce then tested the brakes but got the same rubbing which eased again when I tapped on the calipers.
    OK, Could the rubbing be normal in a rebuilt system ? If tapping on the calipers releases the pistons, could road vibration do the same ? Vibration from the road surface to the wheel to axle, to forks, to
    calipers ?
     
  4. Roast644

    Roast644 Well-Known Member

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    Is the dragging heavy enough that its grabbing the discs? With the front wheel up, doing the spin test you describe, is the wheel noticeably stopping quicker before you tap on the calipers? Your description before the rebuild sounds like there was heavy dragging, is it still the same after all the work you've done? And is the dragging consistent around the entire disc, rather than a warped rotor with a high spot that hits on each revolution?

    Looks like you have made sure the return/compensating hole at the master is clean. And I know this is a dumb question, but I'll ask...both holes in the bottom of the master cylinder are clean? The big one and the tiny one that is smaller than a wire? Is the piston seal definitely retracting past that tiny hole? Brake lever isn't adjusted too tight, and you can see a small geyser from the hole when you pull the lever (with the lid open)? A quick test would be cracking a bleeder valve when things are dragging and seeing if that releases pressure.

    Go for a ride without using the front brake for the last 10 minutes of the ride. Can you feel any heat in the discs after you stop? If no, I'd say you've got things working correctly.
     
  5. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Just remember that while piston expansion (braking) is controlled by fluid pressure being exerted from the master cylinder (hand lever actuation), the caliper piston return is controlled by the caliper fluid seals "grabbing" and pulling the caliper piston back into it's bore.....as the piston is forced out of the caliper, it "rolls/stretches" the rubber fluid seals just slightly, and when the brake pressure is released, it is the "spring action" of the rubber seals that "grabs" and retracts the caliper piston back in as the fluid seals tries to "un-roll" itself back to its static position. The brake fluid "suction" actually provides only a minimal amount of retractive force; most of it is provided by the rubber fluid seals "snapping back" into position:

    https://www.quora.com/What-causes-a-brake-piston-to-retract


    Seals should be lubricated with BRAKE grease (or brake fluid) when re-assembling, and, the seal lands in the caliper body must be "zestfully-clean" of any old hardened gunk or seal debris, as this will distort the seal and affect the "roll-back" effect.

    Also........if you rebuilt the calipers with new pistons, seals, etc., are you sure that the caliper kit was actually for an XJ750 SECA model caliper? Many (most..) vendors sell caliper rebuild kits for "XJ750" calipers and may actually be supplying caliper parts for an XJ750 MAXIM caliper (which is a different caliper), and although the pistons and seals are very similar, they are actually different from one another.
     
    Last edited: Aug 7, 2023
    Franz likes this.
  6. joejr2

    joejr2 Active Member Premium Member

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    I checked with a peice of tape on the top of the tire. Wheels propped up, I spun the wheel by hand the tape showed about 1.2 revolutions before stopping. I was wondering if the pistons
    return to the same spot in the cylinder after braking, or, do they retract the same distance each time as the pads wear ? If they retract to the same position in the cylinders, the fit
    between the rotors and pads would get a bit looser as the pads wear. Am I right or wrong ?
     
  7. Huntchuks

    Huntchuks Well-Known Member

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    The pistons will creep in as the rotors or pads wear. They should retract the same amount from contact with the rotor.
     
  8. joejr2

    joejr2 Active Member Premium Member

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    I tapped on each caliper a little and the wheel spun a bit freer. I wonder if the road vibration on the lower forks would do the same.
     
  9. Timbox

    Timbox Well-Known Member

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    Sounds like it is time to remove the caliper from the wheel, clean them pins up that hold the pads and then flush the system again. I would guess either the pins are sticking (as stated above) or you need to make sure the pressure relieve pin hole in the MC is open and allowing the back fluid to back track to the MC when the lever is released.

    The caliper pins should be very clean and if you have a polishing wheel use it on them. Any burs that are left on the pins is a place that the brakes and stick to and no move as they should.

    Your brake lines have no damage to them correct? Nothing pinched that would hold the flow of fluid back from returning to the MC?
     

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