i had the same problem as cotton with those rubber gaskets and use the leather ones now.
also be sure all the ventholes are in the caps,maybe it sounds strange,but i have 2 militairy eaton (H-D?) caps with the topventhole (fumeoutlet) next to the gasketsurface drilled,but without the lower inletventholes in it!! I'll have to drill them myself.
also sometime ago a friend of mine had his caps chromeplated,somehow they closed off after this, and it took a long time to find out why his bike stopped every few miles!!