Cheers Maxwell. I already have red rubber grease from when I cleaned up and re-greased the slider pins.

I'm really nor sure what's normal or abnormal with the pistons. When I pump the brake with the pads out, one piston comes out before the other, and the "dragging" piston only really comes out easily once the first piston is blocked. This could indicate a problem with the seal, but it might also be normal with the fluid routing? I don't know.

In addition, the pistons are super shiny and look as new, as much as I can see pumping them out as much as I dare. But they really hard to push back in. Also, when they both push back, they stop with 2-3mm of piston still out. Again maybe this is normal? Or maybe it suggests too much brake fluid? But it means that when refitting the caliper with new pads, there's always a little contact with the rotor even before I pump the lever to bring the calipers out. Once the lever is pumped and the pistons come out, they're binding. Period.

I've already bled the brakes a little in case they were overfilled and stopping the pistons retracting fully but to no avail. I guess I could pump the pistons out, open the bleed nipple and push the pistons back and see if it's much easier when the brake fluid is not compressing but coming out the bleed nipple instead?

I have the replacement seal kit here, and am just trying to figure out whether there's something else obvious I should be checking or whether my symptoms to really suggest a complete drain, new seal kit and then refill and bleed.