Is it safe to leave pressure in my line while it is not being used

I have shop air hooked up to my airbrush with the regulator so it doesn't go boom but I have a 60-year-old tank that needs to be resealed my problem with turning the air off is my tank heads a very shitty old valve that you have to spend a little piece of plastic to let air into the line but that thing sometimes it will fall off the screw and you have to fight with it and get it to catch the thread again all while there's a spring pulling it back fighting you that strong as fuck and I can barely get my fingers around it due to its size