Somthing dont sound right there ross, i got at least 5 of these Yamaha taps on bikes and they are as old as shit and dont leak a drop.
you mentioned rubber ? if these are both new units take out the Rubber disk section and soak it in petrol for at least a week. Maybe just maybe there is a swelling you can get in the rubber to pick up the slack.
if you cant get a rubber centre just cut one out of push bike tyre, so long as it has a hole in the centre it will flow juice, or get sexy and use some kind of paper oil gasket. this can be fixed for sure.
Failing that post them to me and ill have a look, if you cant get them to work anyway, you havent lost anything by letting me have a look.