The piston was seized, but with a little heat, WD40 and leverage with a screwdriver, we had movement. Unfortunately, the rubber dust cover got mullered in the process, but such is life. Hooking out the little wire clip exposed a plastic washer and what was left of the dust cover.
This in turn exposed the retaining circlip for the actual brake piston itself. Oof, long circlip pliers needed!
And would you bloody believe it? I've never had this happen to me with a circlip before!
Caused a bit of extra work, but we got there in the end.
So all that was left was to remove the piston, cups and spring. It should slide out easily enough, if not tap the cylinder on a piece of wood a few times. In the picture below you can see the other rubber cup still inside the cylinder.
So that's it completely stripped. The screws for the brake light switch bit the dust, but I'll use a hydraulically-switched banjo bolt anyway, so no problem.
Just need to check the bore and give it a clean out. Although you can buy miniature honers that fit into a drill, I'll show you the poor man's approach. First, take a piece of 1/4", or 6mm, round steel bar and cut a slot into the end with a hacksaw.
Now cut a strip of 400 or 600 grit wet & dry paper and insert it into the slot. Make sure you know which way it is going to turn in the drill.
Wind it round and round, put it into a drill, fire in loads of WD40 and lightly hone the inside of the cylinder.
It should end up nice and smooth inside with no scoring or wear marks - hopefully. This cylinder looks pretty good despite the corrosion on the outside, so a new seal kit should sort the old thing out. Anyone got a 59600-45821 going spare?
All that's left here is to clean up the outside of the cylinder and apply the polish.