Rebuildable Rear master brake cylinders are non existent now. If you look at yours I'm sure they are in bad shape too. The issue is that they are investment casted and porus. If you take your apart you will see small cancer holes like they have rusted. Every single one I've rebuilt have had this issue. Over time this gets worse and sooner or later your will either leak or fail. The cancer will grow or it will abraid the rubber plunger.
I will be resleaving the 5 I have soon and want to offer it to the group if anyone wants theirs done too. I estimate the cost at about $100 if shipped to me disassembled. If I need to tear it down and reassemble add $25 (good time for a rebuild kit).
They will be sleeved in Stainless so that the breaks won't fail in the future.
Please let me know if you'd like yours done. I'm going to do them towards the end of the season, October or November so we can get our riding in. I should be able to turn everything around in a couple weeks time.
Contact me and I'll put you on the list and update everyone along the way.
Fronts will be next.