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The actuator arm on my TRP Hy / Rd front brake no longer returns to the correct position. Even with no cable attached, it requires a bit of help to return to its resting position. The result is that there is a small amount of brake rub.

I’ve used the brake on this bike for two years before now without trouble. I’ve tried to clean out the arm mechanism which perhaps helped a bit.

Is there something I can do to free the arm up? Could it have anything to do with the brake pistons being stiff?

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    Have you had a look at bicycles.stackexchange.com/q/56619/30402 ? Stuck pistons seems to be a very frequent problem with Hy/Rd. This might cause the cable arm to not return either. From what I read, it seems not fixable.
    – gschenk
    Commented Feb 7, 2020 at 0:26
  • Possible duplicate of bicycles.stackexchange.com/questions/56619/trp-hy-rd-adjustment Seems that using the barrel adjuster as a pre-load doesn't work in this brand.
    – Criggie ♦
    Commented Feb 7, 2020 at 2:48
  • Is arms stuck = pistons stuck? I should think so, but I don't know the brakes. It might have to be established in this question.
    – gschenk
    Commented Feb 7, 2020 at 12:05
  • I'm fairly certain the piston must have a spring to return it to its resting position. The piston seals that retract the pistons slightly cannot be responsible for pushing the lever back. The lever must return to it's resting position to connect the main cylinders to the reservoir to allow piston advance as the pads wear.
    – Argenti Apparatus
    Commented Mar 6, 2020 at 19:52

1 Answer 1

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I suggest to clean the whole brake and replace the oil. After some time, it accumulates particles caused by abrasion. The oil of my brakes came out completely black.

First of all, you'll need a bleeding kit for this.

Remove the brake from your bike and remove the pads. Open the top lid by removing both Torx T10 screws. Pour out the used oil and rinse the brake with alcohol, e.g. methylated spirits. Clean the reservoir with a rag.

Now, check if the piston returns the whole stroke by itself.

If it does: Close the lid and tighten the screws. Fill the brake with new mineral oil by help of a bleeding kit.

If it doesn't: Open the body of the brake by removing the two Torx T20 screws on the spoke side of the brake and see what's wrong.

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