You have a very old 880 indeed. That gun must have been made in the early 70’s. Daisy changed that type of “inlet valve” very early on. I have that inlet valve/check valve with the spring in my parts drawer but I am not sure if it will still hold air. I assume that the valve you have has failed. If you want to swap in either of the more recent check valves you will need to swap in the corresponding “guts”. I have done that job in the past BUT I don’t remember all I had to exchange to get it done. I’m curious what type of pump arm retention system your gun has. Is it a small black plastic “C” clip that holds the pump arm?? Welcome to the real fun of this great hobby!