Did you ever remove your A/C?

LEGALLY you are supposed to "recover" refrigerant. Of course if the system -----" accidently "---- develops a leak AT ONE OF THE SERVICE PORTS and the refrigerant all leaks out, uh, well, then there's nothing to recover

On colder days, with the engine cold, there will be less pressure in the system!!!!

At 70 degrees F, R12 is very close to 70 psi