My caliper rotor push oil back in the STI. All of the sudden mineral oil started leaking.

Do you know SKU of this part? (click for larger views)

Unknown part; small plate with raised part in centre and 3 screw holes Part mounted on the brifter, labelled "Shimano Mineral" Other side of part or complementary part from other side.

  • Also - have you used anything but mineral oil in this brake ?
    – Criggie
    Feb 14 at 8:22
  • 3
    Did you try to re-assemble and re-bleed the brakes? Master cylinder should not leak when pistons are fully retracted. The diaphragm from the second photo looks OK and I have hard time imaging how extra pressure would damage it. This is not something I personally experienced, but maybe the system simply had too much brake fluid in it.
    – Klaster_1
    Feb 14 at 8:41
  • @Klaster_1 You can pop them with egregious overfilling, but yea, that doesn’t look to be the case here.
    – MaplePanda
    Feb 14 at 8:44
  • The rubber bladder got a hole by resetting the pistons. And that sucks its a super sensitive part of the lever and it's not w replaceable. Feb 17 at 12:09
  • That sucks. Perhaps you did it a little too aggressively? They are indeed very fragile. If you search it up, you can find hacks like repairing them with superglue, but that would be at your own risk of course.
    – MaplePanda
    Feb 17 at 16:14

According to the ST-7020R exploded diagram at https://dassets.shimano.com/content/dam/global/cg1SHICCycling/final/ev/ev/EV-ST-R7020-4341C.pdf

That part doesn't exist as a removable part, hence why you need to confirm your exact shifter part number.

enter image description here

For completeness - the dealer manual is at https://si.shimano.com/api/publish/storage/pdf/en/dm/RADBR01/DM-RADBR01-07-ENG.pdf and a browse through shows no mention of removing that plate.


I think this is the part you are looking for:

  • Y0C698060 Right hand lid unit
  • Y0C598030 Left hand lid unit

The rubber diaphragms, item 17, have the part numbers:

  • Y0C678000 (right hand)
  • Y0C578000 (left hand)

ST-RX600 Diagram showing these parts highlighted: Exploded diagram of the Shimano ST-RX600 highlighting the above parts

ST-R7020 Diagram showing similar part names on the R7020 unit specifically: Diagram of the same highlighted parts on Shimano R7020 shifter


The sad part is. That part is not replaceable. It's not that tough when you push your piston. That reservoir will pop and mineral oil start leaking. I believe that it should be replaceable but I found no information what so ever. You will need to replace the whole shifter itself.

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