The cap is a 3/4" plug that takes a 1" wrench to remove/ install.. However it won't tighten down. It spins out and while it won't come out, it's loose. Looking at the cap, I noticed the area near the hex face is recessed with no threads. I fixed the issue with a copper washer. Could that have caused it? if there's a vacuum on it because it's sealed so tight? I think there is supposed to be some type of washer but possibly somthing breathable. I bought an extra washer so what I think I'll do is knock a few notch lines in the washer with a chisel so that air can escape and ride it around/ see if that fixes it.