I have a GRX 400 hydraulic caliper and a GRX 600 2 speed front shifter.
- When I pull the front brake lever hard, the brake works as intended.
- 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.
- 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".