The only thing that comes to mind to me is that maybe there is not a good seal at either end of the pressure line or (though not likely) you have a bad pressure gauge. To troubleshoot, if you have a "Mighty Vac" vacuum pump (or some other brand of vacuum brake bleeder kit), it can be set to pump (as compared to vacuum). You could put the pump at end of the pressure line (the sending unit side) and see if you can confirm that the seal at the gauge and the gauge are indeed working properly.
Have you recently added a remote
oil cooler? If you get the
oil lines switched on the remote
oil filter adaptor (the adaptor that goes where the original oil filter mounts to the engine block) you will get no pressure. (Not something you want to do.) Check the lines carefully.