Testing vacuum on a PRV

[revnatscider]

Just bought a new brite tank and PRV. Filled the tank with hot water, forgot about it for a minute, came back to the tank and opened a downspout arm and nearly got sucked into the tank by vacuum. I guess the V in the VPRV isn't working, but I don't know how to test it. Testing P is easy but I can't figure out how to test V other than what I just did. Advice?

[BrewinLou]

Everyone I have ever used can be disassembled. either unscrewed or pushed in and a retaining ring removed. Then all the springs and parts come out. Does it have a dome top or a flat top? The flat tops are easy as you can push the springs in by hand, the bottom is for blow off and the top is for vacuum.

Just for the sake of understanding, you filled the tank with HOT water right? Then you chilled it down? Adding hot water to a tank does not normally cause a vacuum in a tank. Cooling a hot tank down fast will.