Confession: After several tries to get that rubber piece on I made a short taper on the lathe. The widest end matched the diameter of the metal piston shaft and the thin end matched the diameter of the rubber thing. With the rubber piece on the narrow end I placed the taper against the shaft and held it all in the vice. With some brake fluid to lubricate I push the rubber up the taper and it expanded nicely and popped onto the shaft. Then I spotted I had put it the wrong way around so had to remove it and start again. Good to know that an o-ring does exist out there - Thanks kshansen.