First wrap some sandpaper or duct tape around the valve and put it in a vice.
Use a small pipe wrench or channel locks on the cap. It doesn't need to look
pretty on the outside. Like most things in the world it's what's inside that counts.

Parts that help a great deal are.

1- Increase the volume.

Pic of the front of the valve with some of
the threads cut off. to retain strength,
more than half the threads should remain.

Use a 25/64" drill bit to remove about 1/4" from the inside, then polished inside.