Hi Spitfire,
maybe your rear brake master cylinder is in better condition than mine.
I noticed that the front brake master cylinder is the same bore as the rear and the after market repair kit I fitted to the front uses a seal identical to the original Honda seal on the rear.
So it may be possible to buy another aftermarket front kit and swap the seal on the rear master cylinder with the seal supplied in the front after market kit
I noticed on an earlier post you renewed your front caliper piston, I could do with renewing mine as well, because at the moment I can only use half worn brake pads keeping the outer corroded part of the piston away from the seal.
If you still have your old caliper piston please could you measure it for me?
I found a supplier that lists replacement pistons by size.
http://motorcycleproducts.co.uk/catalogue/brake-caliper-piston-kits-c-108_233.html?page=2&sort=2a