Find a commercial/industrial HVAC guy and pressure test the system with nitrogen. You should be able to do a 24hr standing pressure test at 200 psi. Sometimes I'll allow a 3-5 psi change to temperature changes over that time. If you have access to a micron gauge, it should evacuate to 200 microns.
I think you are imagining things. Hoses leak, and oil boils off refrigerant. I doubt you can pull a true 200 microns on an automotive system and get it to actually hold. Just the oil in the system alone is likely enough to trash that
