I would take off the rear caliper bracket (you can't get the wheel out without) and separtae the caliper form the bracket.
Do not undo the tempting slider pin nut to achieve this. That is part #13 and #14. They stay exactly where they are. Just slide them apart.
Clean them up, the should be all nice and shiny. Add a thin smear of silicone or rubber grease intended for hydraulics (eg red rubber grease).
There are two rubber boots. That should be intact. #16 is the most likely one to split. Check it out and replace it if it is. #15 is seated firmly in the hole that is almost out of sight in the picture. There is a red crescent - just to the left of the green slider pin. That (red) boot fits into that.
That green slider pin is fitted into the bit of the caliper that is shaded green.
Clean out the gunge from within the boots.
Gently push out the pistons - just enough to clean them. The pedal will do all three of the rear caliper, but one will move sooner than the others. Find a bit of wood to allow them all to push out and stop at the wood.
Thoroughly clean with a toothbrush and brake fluid. They should be smooth and shiny. If they are pitted, then they may need replacing.
Clean the pad pin #18.
When cleaned up, put it back together. One slider pin is longer thant he other, so it is easy to mate that first. Make sure both are fitted into their respective boots.
Push the nice clean pistons back home. Make sure you have not dislodged the pad spring in the roof. If you have seat it properly (the wider strip sits closest to the pistons)
Put the caliper/ bracket back onto the axle so that it doesn't straing the hydraulic cables.
You could do the same with the front calipers - but the front left is difficult to separate as there is a very short hose connecting the bracket and caliper. Apparently it is possible. I've never tried to do it.
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Check the operation of the SMC - careful not to push the rear pistons out.
Clean up the needle roller bearings in the front left fork leg - the caliper mounting bracket. Theres a chromes bush that will push out. The ends are often encrusted, so clean that up before you push it through - check the bearings are rolling OK and grease.
Get into the swing arm and check out the brake hoses that feed to the back brake. There re a couple of unions that might need a clean up and a protective brushing with ACF50 or some other protective oil substance.
Check / replace the rear final drive oil. Be careful with the larger cover - it is more fragile than it looks.
Check wheel bearings for smooth running - tick you finger in and twist the bearings around. You are feeling for roughness, notchiness, free lateral movement. Do not spin the wheel on your fingers. If the bearing is binding, you will end up with a broken finger, or you will be doing a bike-wheel around the hub (as opposed to a cart-wheel). So please set up a video recorder before you do that !!