Cone Clutch - How to Stop it Slipping

I have a cone clutch in the gear box/transmission of my mower. The cone is steel and the cup lining is brass. The clutch slips and I have been trying all sorts of ways of ways of preventing this including roughening the cone (bad idea - it worked initially but only for a short period). After a year of cutting, I have dismantled the gearbox again. The cone and cup have become nicely smoothed. I tried using grinding paste to smooth out imperfections and the surfaces are now dead smooth. I checked this with a steel rule back illuminated with a light. It's not worth buying a new clutch and anyway I am on a "mission" now to make this work!
Any ideas on what I can do?

Edit: I have discovered lots of threads on tractor forums about lining clutches with leather. So I will give this a try.