The rear calliper are more complex than the front, which can be serviced, with care, using standard tools & a liberal amount of concentration plus a healthy dollop of that care I’ve just referred to; your life depends on it being done correctly.
The rears combine the standard piston system with the handbrake & an automatic handbrake adjustment mechanism.
These can be serviced on the bench, all the parts are readily available including the needle roller bearings in the leaver assembly in the eccentric chamber; it is this chamber that is the main problem, it fills with water & all the grot that is carried by the water.
Do not listen to anyone who tells you that WD40 will cure the problem, it won’t. It will expel moisture but it leaves behind a sludge & compounds the issues
The callipers can be cured only by taking the lot to pieces & rebuilding them with new parts.
However you need long pronged circlip pliers, which were available but we’re not cheap, I can’t see that situation having changed.
Everything has to be bob on as you don’t want half a day out with the undertaker plus once you have avoided this you want many more MOT passes along the road.
If you decide to rebuild, one tip, use a compressor to blow the piston out of the callipers, always use new rubbers, use the proscribed grease & cleanliness cleanliness cleanliness.
M