Out of interest, as it seems the seats have been the problem the whole time, perhaps a good test would be to set up a test rig that will hold a primer cap and have a sizable tank above creating decent head pressure.
Then ream the seat and put a completed cap in the apparatus with an old plastic needle. If the bore of the seat is smooth enough to decently seal on a plastic needle, it will seal great on a viton tip and in theory would have no burr to catch the rubber
