I got another front brake master cylinder off eBay.  The size this time?  Back to the original 1/2″.  Increasing the size of the master cylinder is a big mistake, don’t do it.  Instead, replace the caliper seals and then, if you still have problems, consider Dot 5 brake fluid (but satisfy yourself that this is a good idea).  There is a stamp on the reservoir saying Dot 3 or 4 only and the replacement caliper seals probably say something similar.  I’m keeping an eye on things but my seals do not appear to be dissolving so far.

Changed the front brake pads in both calipers, too, and finally have pads for the rear.  What with the two new tyres, Silver is going to be like a new bike.