Super Pista Pump Rebuild Problem

I just put a new leather pump washer in my Silca Super Pista floor pump. It
pushes out air much better now but on the upstroke I am pulling against a
vacuum until the pump washer passes the vent hole near the top of the pump
tube. I cleaned up the check valve which seemed fine (and should not be
linked to this?)
Does the washer just need some break-in such that I do not get such a good
seal on the upstroke or is there some other problem involved here?
Thanks,
Barbour Warren