Question Dave - (and you may have already stated the reason - which I missed) why would you discard the second disc? Many folks over here would give their eye teeth for that set-up.
The brakes were completely jammed on. The brake lines were the original fittings so would need replacing. There's four bits of brake line, plus a splitter which incorporates the brake switch. Then there's some steel pipe (which was OK but the fittings were a bit rounded, I would have cleaned them up).
So I started in on the getting the callipers apart. For that year the calipers have pucks on either side and when you pump to free the calipers, one side pops out and the other side stays put. I tried holding one side in with a clamp but couldn't find anything to fit. So, I started in on the other side (a little wiser now) and cobbled some bits of mild steel and threaded bar to hold each puck in in turn.
That's when the master cylinder started leaking badly. I have a 1975 xs650B with a Nissin 13/16" bore front master cylinder. There's a previous thread from about five years ago from someone else now discussing this. Seems like the 13/16" rebuild kits aren't available. So now I'm "in" for a new master cylinder and still have two half disassembled / half jammed calipers.
By now the cost of rebuilding both sides in the original set up was starting to rack up. I was going to have to buy bits for two complete sides. Just the shims that go behind the disk pads are £15 a set. And I was getting nowhere fast. I thought I would rebuild just one side and but was now going to have to buy a new master cylinder (or risk buying second hand and refurbishing that). I thought that I could refurbish and spend out on the two disk setup some time down the line.
The catch 22 was if I buy a caliper and use it on one disk I either buy something that is too big for one disk and will be later too small for two, or vice versa. I either end up with an incompatible system or more expense down the line.
Everyone here seems to replacing oversized master cylinders with smaller "pit bike" versions and installing cheap straight through brake lines. That settled it for me.
@5twins advised on the technicalities
Early (1974 / 75) late (post 1977) brake calliper compatibility concerning most standard stuff being oversized in any case, and it was decided.
Later I found Yambits did a suitable RD125LC brake lever and cylinder (with the required smaller bore) for about £50. (Brand new. Bargain) (And by now my eyes were turning square at looking at Chinese websites.) and later still I found they had a remanufactured 75 caliper. £105 for the complete calliper, banjo bolt, crush washers, brake pads and shims.
It all sounds very complicated in hindsight.
I still have all the bits.
Dave