I had the same issue but ended up getting my rear MS to work using the original parts. All the rubber was in good shape while the metal was quite corroded. After cleaning up all the metal as best I could it still didn't want to work. The plunger was sticking and not springing back like it should. So, I took a wooden dowel and wrapped some 0000 steal wool around the tip and taped it on with masking tape. I secured the dowel in my drill and wet the steal wool and reamed out the bore really good. After that, it's been working perfectly.
I'm convinced the rear MS parts just no longer exist. So, it's get the old one working or mod in a replacement. Luckily, I got mine working. Good luck!