The ones I've seen were just shrader valves like those that go in a car tire. Screw it out with a valve removal tool or a metal valve cap that has that feature on the end. You then have to charge it to the correct air pressure.
And for Bob, the OP, know also that just because the tank loses that pressure, it doesn't necessarily follow that the valve is bad. In older tanks, without rubber bladders, the air is "absorbed" by the water and the ambient pressure of the tank decreases with time until there is no air cushion at all. In those types of tanks, routine re-pressurization is required. Same holds true for a bladder type tank that has a ruptured bladder. I didn't see where you explained just why you feel the valve itself is the problem.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.