testing oil pressure switches

[Nicholas Lawrence]

I'm testing four oil pressure switches, one 1.8 tests ok per procedure in
Bentley. (I'm applying air pressure with an Oxygen tank and regulator.)  On
three of the low pressure switches I can't measure any continuity with or
without pressure.  Two were from running cars, third is from a new never
installed engine.  I know it's possible all three are bad but does anyone
have another idea.  TIA

Nick