I pulled apart the heater box to replace the heater core last winter and I know the door isn't binding up. I have a strong feeling it's the rubber grommet that seals the rod to the canister because it works perfectly pulling the door to the open position but has some trouble pulling it closed (pushing the rod out of the canister) and I can hear an occasional "hissing" sound although the vacuum lines are good. I'm gonna try slathering some o-ring grease on the rod (I feel dirty typing that lmao) and see if that does the trick, it's not quite broken enough to be broken lol.