You were trying the right things. There’s nothing out of the ordinary with the B50 in terms of filling. Very conventional hammer and knock-open valve. It’s very nearly a verbatim copy of the Daystate Huntsman.
Yes, I would say the next step is go ahead and disassemble. Remove the valve and inspect the poppet. If it has ever been fired with no air, there is a decent chance the hammer overdrove the stem, causing the valve spring to go to coil bind. The poppet therefore suddenly hits a hard stop while the hammer is still driving the stem, which has the potential to drive the stem through the poppet, destroying it.
Note that once you get the fasteners out, the valve may not budge. The two halves of the valve are typically threaded together such that they bulge the O-ring and wedge the valve into the tube. So it may be necessary to loosen the front half of the valve a turn or two before removing it. For this task, any of the following can be used: a loooong flat head screwdriver, a piece of flat bar stock, or a sacrificial socket (sides filed away to leave two teeth at 180deg apart).
While you’re in, inspect the O-rings for signs of aging or extrusion, or go ahead at do a reseal job if that option is available to you. At the very minimum, I strongly recommend replacing the O-ring on the fill port check valve with a -006 in 90 durometer. It is a common source of leaks because the gap around the check valve is so large that 70 durometer tends to extrude and fail.
