[68 C10 Driver]

If you have access to nitrogen, pressurize the system with that, do not use compressed air as that will introduce moisture into the dryer unit. If you do not have nitrogen, then charge with 134 and dye and use a sniffer and black light to look for leaks. Check all connection points and any original components that you are reusing. The condenser or evaporator may have leaks that the sniffer or dye should expose. I wouldn't dump the oil out of the compressor, it should be fine and if under warranty changing it out may void that compressor warranty.

I have a EPA Section 609 (motor vehicle air conditioning) certification, just so you know I am not just throwing out Google facts. In my experience most times a system will not hold vacuum is because an O-ring is damaged, old component is leaking (i.e. condenser has a pinhole due to rock damage), or the rubber lines were not changed when converting from R-12 to R-134A. R-134A molecules are significantly smaller and will permeate R-12 line sets.