Jet pump won't shut-off. I just preplaced the pressure switch because it was leaking and now the pump builds to about 20lb and the pressure gage starts to bounce around. When the pump is unpluged the pressure in the pressure tank drops to 0. Pressure tank is 4 years old (air at 23 psi). I tried to adjust the pressure in the new 30/50 switch but I might not be doing it correctly. The pump used to build to about 45lbs then shut off and the switch is all that I changed.

Does it sound like I should have gotten a 20/40 switch or is there something else I have to do to get the system working correctly?

The bladder tank setting should be 28 PSI for a 30/50 control box.
I would give a close look at the check valve. It sounds like it's not holding the pressure once it's built up. Regards, Tom

Look inside the cover of the pressure switch and it should give precise instructions on adjusting.