Thanks for the response mrtappan,
After talking to my friendly Honda dealer for about half an hour late this afternoon; he indicated he could not read the number clearly on the reservoir, but it looked like 5/8. Short version, it appears the 1800 Valks front brake MC is 5/8. Which is JAP shorthand for 15.8mm.
Figured this out by getting a Honda part number for the rebuild kit (45530-MA5-671), then entering that into ebay to see what bikes matched that number. Turns out a VF1100F Sabre (among many others) uses the same part; and the Sabre in my shed is marked as 5/8.
I was looking at trying to find some black MC's for my 1500Valk - not sure I want to alter the stock MC size...

Sigh.