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I have a GRX 400 hydraulic caliper and a GRX 600 2 speed front shifter.

  1. When I pull the front brake lever hard, the brake works as intended.
  2. If I pull it slowly, or repeatedly tap it, then I lose power. Specifically, if I pull the lever just enough to hear the brake engage and wait, the brake eventually disengages. I can repeat this process until the lever is all the way to the bar and the wheel moves freely.
  3. As soon as I release the brake and pull the lever again, it goes back to full power.

It seems similar to the "ghost leak" issue described in a comment on this thread except that it doesn't happen if I maintain enough power with the brakes. So I don't think it's due to a hose issue. I have inspected the external part of the hose, and don't see any issues (although 80% of the hose is either under the handlebars or in the fork).

Based on my rudimentary understanding of hydraulic brakes, it seems like there could be a sort of issue with the piston/reservoir; for instance an o-ring on the piston not engaging properly. But I don't know enough about how the GRX levers work to say more.

Any ideas what the issue could be? The bike is 2 years old, but has <10 miles.

EDIT: I went to the bike shop today and the mechanic said it was "the weirdest thing he's ever seen".

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  • 2
    You say they're hydraulic, but then you mention "cable". I assume you mean "hose"
    – Chris H
    Commented Oct 14, 2022 at 14:06
  • yes, the hose that connects the lever to the caliper
    – overfull hbox
    Commented Oct 14, 2022 at 15:33
  • Would you say your pad to rotor clearance is about normal? Or have you had issues with brake rub lately as well? My suspicion is that the brake is slightly overfilled and it's starting to engage before the reservoir port fully closes.
    – MaplePanda
    Commented Oct 14, 2022 at 21:58
  • I just got the bike, but haven't noticed any issues with the braking (there is a very slight rub, but I haven't properly centered the calipers).
    – overfull hbox
    Commented Oct 15, 2022 at 0:01

1 Answer 1

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The lever has a reservoir that the fluid in the hose is free to flow in an out of when the lever is released, this allows automatic pad placement to account for pad wear and heat expansion of the fluid. When the brakes are applied, a valve shuts, blocking fluid from traveling out of the brake line into the reservoir.

It sounds like the valve is not closing properly when the brakes are applied lightly. A lever service is possibly required, however increasing the free stroke (and adjust reach to suit) may provide a solution.

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  • Is there any way that I might be able to "reset" the valve without replacing the lever?
    – overfull hbox
    Commented Oct 15, 2022 at 0:16
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    I am not familiar with them, Shimano manuals here. si.shimano.com/en/um/07X0A
    – mattnz
    Commented Oct 15, 2022 at 1:54
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    @tch Try bleeding the brakes to verify that the correct oil volume is in there. The valve itself is an entirely passive component (a hole on the side of the master cylinder that the piston passes over). There's no resetting or anything you can do about it.
    – MaplePanda
    Commented Oct 15, 2022 at 14:24

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