I had a stuck bottom cap on my cornet and I tried PBBlaster applied with a medicine dripper (the can would spray far too much). It wouldn't budge after many attempts of moving it without a tool. Finally as a last resort I took the cut off finger of a worn out leather glove and slipped it over the cap to protect it, and then used a Channel Lock or water pump pliers very gently and was able to loosen it. I put very little force into squeezing the pliers- remember, brass is a very soft metal and the pliers handle gives a lot of leverage.

I had a stuck top cap on a mellophone I bought and I was able to loosen it after tapping it with a screwdriver handle covered with the cut off finger of an old glove.