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  1. #1
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    Pistons leaking on Shimano M755 4-pots??

    I checked my front brake the other day because I felt the braking power was not there and I was hearing some rubbing -- found what looks to be 4 - dots of mineral oil on the back of the pads (2 on each side - were the pistons contact the pad). I cleaned it up and re-lubed the pistons with mineral oil. The braking power was still not there so I changed out the pads and it came back -- but I am worried now that the pistions have a slight leak.

    I have put the contaminated pads on with the pad spacer - and clamped the brake up overnight -- is this a good enough test as to whether or not the pistions are leaking? I plan to see what I find after work tonight.

    So I am wondering:

    - Any rebuild kits out there with new seals?
    - If I relube the pistions with a 100% silicone lube (brand recommedation) -- could that help seal them better
    - Is it possible I have the reservoir too overfilled?

    Thanks

  2. #2
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    There were build kits availible. It might be a bit of a search to find them now as Shimano hasn't produced the M755 for 4 or 5 years.
    Get over it!

  3. #3
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    This suggests there parts are not available for the calipers, only the lever: http://techdocs.shimano.com/media/te...9830610843.pdf

    If the piston is leaking, different lubrication shouldn't make any difference. Overfilling the reservoir shouldn't make any difference: it will just spill out at the lever.

    I have always thought that if mine started to leak I'd pull them apart and take them to a car brake specialist to get the seals matched up. The other alternative is a bearing supply place: they usually have an excellent stock of seals. Just like any caliper, I expect that they will ultimately wear our, but using mineral oil rather than brake fluid this should take much longer as the oil doesn't attract water like the fluid.

    Good luck.

    Tim

  4. #4
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    more info

    Called a few places trying to locate some seals -- no luck to far -- I heard the same thing from them -- everything but the caliper has replacement parts. Talking to shimano tech -- they said they rarely if ever hear of leaking pistons -- he suggested bringing the pistons out as far as possible without popping them out and cleaning and re-lubing with mineral oil - hopefully clearing away some dirt in the seals. I will see how that works. Shimano tech also said that since the brakes use mineral oil the seals should last a long time -- mineral oil is not a corrosive and eats seals like the DOT brake fluid.

  5. #5
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    another update

    looked more closely. Since I lubed the pistons so liberally I think I got a lot of mineral oil behind the black inserts into the pistons (these inserts into the piston help to islolate the piston from taking on the heat that is generated from the pads -- at least that is what I think they are for). So I tried to flush those inserts out the best I could -- the reason I was getting 4 spots on the back of the pads was because the excess mineral oil behind the inserts would be forced out with the pressure put on the pads during braking.

    Now I will say that when clamping the brake lever in the stand and watching the pistons extend from the caliper body as the pads contact the pad spacer -- I do see in one or two places a small small pin hole size bubble as the piston extends out -- not sure if that is an issue or not. I tried to check the same thing for the rear but it is tougher to view when clamping the lever.

    Since I feel I flushed things out really well -- I have a good pair of used pads that I am going to put on see how things work out. Keeping my fingers crossed!

    Any thoughts on the pin hole sized bubbles?

  6. #6
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    yeah I've got julie brakes from '01 and I'm wondering how to replace the seals?

  7. #7
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    Quote Originally Posted by eman2
    looked more closely. Since I lubed the pistons so liberally I think I got a lot of mineral oil behind the black inserts into the pistons (these inserts into the piston help to islolate the piston from taking on the heat that is generated from the pads -- at least that is what I think they are for). So I tried to flush those inserts out the best I could -- the reason I was getting 4 spots on the back of the pads was because the excess mineral oil behind the inserts would be forced out with the pressure put on the pads during braking.

    Now I will say that when clamping the brake lever in the stand and watching the pistons extend from the caliper body as the pads contact the pad spacer -- I do see in one or two places a small small pin hole size bubble as the piston extends out -- not sure if that is an issue or not. I tried to check the same thing for the rear but it is tougher to view when clamping the lever.

    Since I feel I flushed things out really well -- I have a good pair of used pads that I am going to put on see how things work out. Keeping my fingers crossed!

    Any thoughts on the pin hole sized bubbles?

    How did ya make out
    [SIZE="4"]Thank God men cannot fly, and lay waste the sky as well as the earth.
    Henry David Thoreau[/SIZE]


    http://vimeo.com/6232260

  8. #8
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    Also, I forgot to mention You prolly can get build kits for the M755 (for future use, just in case - if you keep them eventually you will need/want to overhaul the calipers anyway) http://www.bikeparts.com

    Good Luck! Been running my 4 pots for 4 years and not one prob
    [SIZE="4"]Thank God men cannot fly, and lay waste the sky as well as the earth.
    Henry David Thoreau[/SIZE]


    http://vimeo.com/6232260

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