[pubsinger]

Might be worth applying gentle pressure to the brake pedal, with the pads removed the piston has plenty of room to travel further out, might be enough to get it moving. Don't allow it to move too far out or you'll never get the rewind tool in.

You should be able to lift the rubber dust boot/cover and squirt some Plusgas (other freeing agents are available) into the caliper. Once you get it moving wind it fully in, brake pedal out, wind in, keep repeating till it's operating smoothly.

All the best

Pub

 

[pubsinger]

Is the rubber sleeve just there to stop dirt or is it a fluid seal too?

Just there to stop dirt getting it, you can ease the rubber away from the top of the piston and squirt (for example) plusgas into the caliper.

How do you actually get the old sleeve off and securely fit a new one?

Can't help you with that one, never taken one apart. If you got a one where the rubber is damaged you're not going to lose anything by being heavy-handed with it, rip it off, may find it just sits on/under a lip or held in with some sort of clip.

Good luck

Pub