From your decsription your anti-vacuum valve is stuck closed and you are experiencing the "false pressure" indication that results from having a stuck closed anti vacuum valve, & where the operator is not following the practice of bleeding all the pressure out of the boiler upon switching off by opening (and leaving open) one of the steam taps, and making sure the tap is still open when switching the machine back on...until it starts to leak steam from the end of the steam pipe....before closing the tap.
The fix is to:
a) tap the nipple on the anti vacuum valve with the end of a screwdriver to unstick it and
b) always follow the practice noted above, of bleeding off all pressure when switching off and back on etc etc etc.
The OPV on the boiler has nothing to do with this, its just a safety valve that *blows off at a pressure way above normal working pressure.
c) If you are tapping the anti vacuum valve while the machine is pressurised, make sure you dont burn yourself
d) before lifting the lid on any esp machine dont just switch off, UNPLUG it from the wall....dont take any chances with elektrickery.
Hope this helps.
aka FC, first / original CS site sponsor. *