Replacing pressure (diaphragm) switch on sewage pump

This weekend, possibly beyond my scope of ability, I tried to replace the pressure switch on my sewage pump (Hydromatic skv40). I got a new Hydromatic pressure switch from a local plumber's parts dept - looks just like the one I'm replacing. I thought all I would have to do is remove the old pressure switch and attach the new one. After getting it attached and piggy backing the pump cord/plug, I filled the basin with water, but the pressure switch never kicks on. I can reach in and push the bottom of the switch and the pump kicks on, but it isn't firing by itself.
This was the same problem with the one I am replacing. The pump works fine when I plug it directly into the wall so I assumed it was the pressure switch? How does the pressure switch work? Does the basin have to be air tight to create enough pressure?
-Thanks -
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