Sounds like you are on the right track, I would also flush the system and add the correct amount of oil especially if you don’t know how long it has been setting open. While you are in there you might want to replace some of the other old o-rings (cheap and easy to do now). Also look at the AC fittings and lines and look for old dirty oil gathered on the lines that can also be a sign of an AC leak. Repair or replace as needed and enjoy the cool breeze