Release pressure in the cooling system by removing the cap, then reinstall
Remove the 4 screws fron the top of the watervalve, and pull out the insert by pulling up on the center of the magnet (the silver "knob" on top. (Oh, remove the wiring connector first).
Examine the part you just removed, on the underside. It's not the small central rubber seal that goes bad, but the diaphragm under the center seal. Pull out slightly on the center seal and look for any rips or holes in that larger rubber bellows or diaphragm.