I have used brand new Yellow Jacket 2 stage 5cfm pumps that would not get to 25 microns. An older auto ac system evacuated to 100 microns would be very good. The ambient temperature is a consideration, the higher the better. On critical systems I have run a torch along all the available exposed piping just to flash more moisture into the vaccum. Not possible on a auto ac system, but worth a mention in regards to temperature.