Terrible Front Brakes

[Grimly]

So can anyone point me in the right direction for a 12mm master cylinder for my ‘75 in the UK please?

What front end? The large single disc or the twin Euro-market disc setup?
For the twin disc I dumped the original for a 14mm ex-Kwak m/c but even though it provided fantastic stopping power, the lever came too far back. I substituted a 5/8" from a Honda NTV700 Deauville for that and found a good compromise.

 

[AlanW]

What front end? The large single disc or the twin Euro-market disc setup?
For the twin disc I dumped the original for a 14mm ex-Kwak m/c but even though it provided fantastic stopping power, the lever came too far back. I substituted a 5/8" from a Honda NTV700 Deauville for that and found a good compromise.

Single disk US spec

 

[5twins]

Then I think you want a 14mm MC, it's said to work very well with the single caliper w/ 2 pistons. And there are lots available from '77 and newer 650s. That's the size that came stock on them.