If you are sure of the component assembly, a drilled and parted chunk of wood is a useful vise adjunct.
I'm sure you are dealing with "difficult", not frozen nor impossible.
In fact, if "frozen" is the best descriptor, try a heat gun on the junction. Any movement only aids dissassembly.
I have a pair of the AR6, one in .17, and one in .22. Great "six shooters"!. The fill check valve on both have needed frequent attention, though I have never had difficulty breaking down the reservoir cap.
As suggested. MAKE SURE THERE IS NO RESIDUAL PRESSURE! Drop the hammer, press it home, listen for the final fizz...
|This message has been edited by CalG on Nov 12, 2017 10:10 PM|