Mike, the breather cap has a ridge near the bottom to keep it in the rubber bushing/gasket/whatever. When the cap stays in without being removed for a very long time, it hardens into place, and the ridge on the cap is hard to pull thru.
Most of the breather caps I buy have a new rubber "sealer" that you just take out the old one and put the new one in to replace it. Made to fit the new cap.:nanawrench:
take a small screwdriver and drive it through the rubber to split it and the tube will come right out .they will get hard as a rock in time and not let the tube come out easily. or just keep pulling on it .either way you will need to break up the old one to get it out of the valve cover. autozone sells the gromets in thier help section. i buy mine there