electronic smith oil pressure gauge

[ERA 626]

Here is the issue:
I have a brand new electronic Smith oil pressure gauge I got from nissonger.
I wired it up exactly per instructions, This is a new motor build so there is no oil in the block it is just a mock up at this point.

When I turn on the key the gauge goes to 95 PSI and stays there. I tripple checked the wiring ( hot, ground, sender input) it is all good.

I called nissonger and he told me that it could be a grounding issue to the sender, this makes no sense to me because the gauge has a ground wire running to the sender, so I also grounded the sender to the chassis ground, same issue. Same result if I disconnect the sender, so to me this rules out the sender.

I had him send me a new gauge, same thing. So I turned on the key and applied an air compressor to the gauge to see if I get a reading, when I do this the gauge goes to 100psi then back to 95 PSI... Is this normal? I need advise on this.