SW20 1990 rear brake caliper piston replacement
My left rear caliper handbrake cable had seized and the rear piston and disc badly overheated and were junk. When I looked for a replacement piston they are not available anywhere. The only option was a kit from the UK that included a new piston body and a circlip that can be used to retain the guts of a donor piston, Ebay "Rear Caliper Repair Kit & Pistons (41mm) For Toyota MR2 SW20 2.0 (1989-1991)". The guts of the donor piston are a screw and spring mechanism that form part of the handbrake action. In the next image these allow you to rebuild the "piston". This would have been a single integrated part from Toyota.
You also need a plate that comes from the donor piston. This is retained in the donor piston by some crimping of the inner wall of the piston. I decided to make two vertical cuts down the wall of the donor piston and a horizontal cut across the face of the piston so that I could split it into two and remove the guts. I didn't know the inside layout of the guts of the piston and didn't realise that there was a spring in there and I was lucky to not cut it. I think a better option would be to find someone with a lathe and ask then to do a cut around the waist of the piston at the level of the retaining plate so that you were just under the level of the crimps.
On the reassembly, the piston internal spring, plate and the new circlip is easy to fit however there are a number of subsequent items that need to be loaded into the piston, the "adjusting..." bits below. And then you fit the snap ring. Toyota mention a special tool to compress this assembly to get the snap ring in. I found that I could put a bit of brass tubing into the chuck of my drill press and use this to load up all the items into the piston and then add the retaining circlip. The tubing fitted over the adjusting bolt and was not too big so that I could get the snap ring around it. It turned out to be pretty easy.
The biggest improvement was that I was able to get two new brake cable assemblies from www.nengun.com
in Japan for A$121 for the pair and the handbrake action now feels great.
I did have to fit new discs at the rear as the left one was junk.