To prevent any possible external source of ignition and/or moisture form getting into a freshly loaded bore through the vent I do the following to 100% seal it:
I bought a roll of the type of duct tape that does not leave any sticky residue...GI OD matte finish dry adhesive duct tape, Gaffer's duct tape, Grip duct tape, etc.
When I load in the garage at 4:30am before leaving for hunting, I slip on the leather hammer stall, and press a 1/2" wide strip of this dry duct tape across the pan, then flat up against the vent on the breech flat, then over the top flat, etc.
Get to my stand, peel off the tape and stick it on the stock for use in knapping a flint, or for the return ride home, prime the pan and start hunting. It's a simple, quick, 100% reliable insurance policy against accidental ignition through the vent...a modern concession I have no problem making when using my Flintlocks.