I just replaced mine because I spotted the start of a fluid leak.
There are no oem parts available to service the rear master cylinder. My bike is a '97 and I've already made my mind up that clutch and both brake master cylinders will be replaced if they give trouble (because they have been very reliable). I'm ignoring boneyard replacements because they could be of similar age and wear. The master cylinders get a lot of use so when the rear started leaking, I decided to go with new oem.
I shall dismantle the used unit and might try to source a new seal just for a temporary spare. But if the new replacement lasts another 13 years, then either me or the bike may be long gone!
You could try removing the circlip and pull the insides apart, but being an important brake part perhaps a new oem is the best route. I think you will be paying around $140.But if the master cylinder is not returning and you burn out pads and a rear rotor, the cost will be much much more.
...... I've now partially dismantled my old rear brake master cylinder. Most of the sealing is done by two seals, the usual cup type pump seal and a second flat cross section circular seal in the master cylinder bore. The seal doing the work for braking is the cup seal.
The piston itself slides in a ptfe liner which whilst easy to press in, looks difficult to get out. The piston looks as though it's made of cast ali and could corrode if there was water trapped in the rubber boot. I've always filled my boot with grease. If yours is sticking it looks like it could be betwen the piston and this ptfe liner. You might get lucky removing it and cleaning out any gunk, but if the piston has signs of corrosion - forget it and buy new.
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