Intro: Vacuum and High Pressure Pump
Introductory primer on converting a refrigeration pump into a high pressure air compressor or vacuum pump. If you are planning on doing this you need to evacuate the refrigeration unit in a responsible way. That is beyond the scope for most people. I would look to appliance repair places for a pump. They have to evacuate anyway to dispose of old units removed from customer's homes. Tell them you are wanting to build a vacuum pump. Cash talks.
You also must heed my warning on using a pressure cutoff switch and an over pressure protection valve. I can not stress this enough. Left on and running with no protections this pump could generate over 400psi, way beyond the rated pressures of most tanks, piping and hoses. If you do not have over-pressure protection it is a bomb. Oh and NEVER use pvc piping, even if it says it is rated for the pressures. When if fails because of age, defect or impact it send out shrapnel. Use hard copper or rubber or other rated lines. I have been hearing of people using pex water line to run less than 150psi air. That might be worth looking into.