If anyone is interested, and for reference, I email trp and got this reply:
Thank you for the email. Other than a bleed and a piston exercise, I don’t have many other suggestions. Exercising the pistons:
Start by removing the pads
Use a tire lever, or box-end wrench, or something to hold the freely-moving piston in place
Actuate the brake to get the sticking piston to move out slightly - no more than 3mm, as you don't want it to pop all of the way out.
Use a Q-tip and mineral oil to clean around the piston. The mineral oil will also act as a lubricant.
Use the tire lever/box-end wrench/etc. to push the piston back in.
Hold the other piston in again while actuating the brakes to push the sticky one back out.
Repeat 5 & 6 a couple of times. Do the same with the other piston, so they're both starting over clean and lubed.
Bleeding: this is essentially a mtb brake, without a hose, and the lever assembly fused to the caliper. So your bleed will be very similar to this: https://www.youtube.com/watch?v=cS0pBCBT65g
(I don’t have a current bleed video for the Hy/Rd – coming soon!)
Going to try this out and see how they perform. Will probably use shimano brake oil unless anyone knows why this would be a really bad idea, as I'm going to buy a big tub and do the MTB brakes too.
Good shout. Just a thought, but what happens if you do pop the pistons out? Are they easy enough to get back in? Was thinking it might be worthwhile to remove them completely, clean and check the seals, then reassemble and refill.
If anyone is interested, and for reference, I email trp and got this reply:
Going to try this out and see how they perform. Will probably use shimano brake oil unless anyone knows why this would be a really bad idea, as I'm going to buy a big tub and do the MTB brakes too.