TRP Hy Rd brake : why won’t actuator arm return properly? 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?
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.