Originally Posted by cjn79
Or you could have your plumber replace the valve for you.
I think you are right, it some kind of relief valve. The top part of the valve has a calibration scale etched into it and the name "WILKINS
" etched in. It has marks like 1500 then lower marks of 175, 150, 125. All those marks are etched on a nut that looks like it could be adjusted with a T screwdriver vertically from the top.
Going to need a plumber on this one.