3.0L Engine Vacuum Question

I started my 99 Ranger tonight and saw that there was no oil pressure reading on the gauge. I shut it off and on several times with no reading. I then checked the oil and it was fine. in doing so I opened the oil filler cap. Then i started it back up and got a reading. the gauge measured half way. Not sure why i did this but as it was running I open the oil filler cap and the engine shut off due to the big vacuum leak.

Is that vacuum normal for the 3.0l, i have asked a few people and was told it may be depending on how ford did the vacuum for that engine.


Oh I am planning on replacing the PVC Valve and Oil sending unit.


Thanks


Ray