Well, it sounds like the pressure switch is working properly.  I'm stumped as with a pressure switch there really isn't any 'calibration' to do.  Everything is working as it should back to the point where you read voltage going from 13.8 to 0.8.  Is there any other part of this?  Like a recent reset/reprogram of the SmarTrax controller?  Which type of controller do you have?  Is it a rectangular box or a 'node' with diagnostic lights?

FYI when reading resistances for something like this, you'd want to be set to 2K, not 2M.  But that's OK, based on the testing you just shared, I don't think you need to measure resistances any further.