I am no expert but Andy has posted pictures showing badly deteriorated center seals. It may be a common problem so there is a good chance your center seal needs to be replaced. If i have the crank out of the motor there is no way i would put it back in without rebuilding the whole thing unless I could be sure all components are in top condition. I would think the centre seal is important for all engine operation as it prevents leakage between cylinders which would be just as bad as leakage outside the crankcase.