Please don't use straight Propane in an R12 system. The appropriate blend to replace R12 is about 40% R660a (iso-butane) and 60% R290 (propane).
In this chart OZ-12 is the 40/60 blend. Watch the pressures. Because I like a perverse mix of country units, Y axis is pressure in PSI and X axis is temperature in degrees Centigrade.
Not butane. Iso-butane.