hi mick,first,is this with the alloy rocker box i sent you?...if so it MUST have that baffle in.
if it has ,or your still with the tin one,it sounds like you may have 'blow by' past the piston rings,pressurising the crankcase too much.
you cannot block the vent,both pistons decend together and the cranckase needs to vent up through the pushrod tubes or it will blow oil seals/gaskets................
it is possible you have too much oil in?.....thats a clutching at straws one tho......