I have had a number of those taps over the years and found that they all eventually became too stiff to opperate without carrying a short tube for leverage. I don't believe they were ever factory fitted and to my mind are better suited for use as a gas tap( thats natural gas ). The other problem is that any foreign matter ie rust etc will act as a grinding paste and create anular ridges btn the working surfaces. Lapping with grinding paste is often suggested, but I think thats a waste of time.
I wont bore you all (again) with the problems I had with my Indian made tank, suffice to say it was too high causing the same probs as yours etc. etc.
1960 Matchless G12
1958 AJS 18s