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Generally, conversion from R12 to R134A is not an issue.
I accept a systen capacity loss of roughly 2% on a good system.
Converting back to R12 is not as easy, flushing is needed, all seals and O rings must be replaced before proceeding..
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Something is wrong here!
R12 is fine with the old fashioned "O" rings. R134 has to have those green thingies. Changing from R12 to 134 requires replacing the "O" rings, going back to R12 does not. R12 is fine with either type of "O" rings.
R12 is normally used with mineral oil. All the oil has to be removed and replaced with synthetic when changing to 134. The synthetic Ester does not mix with mineral oil.
I think the preasure is sopposed to be 25 lbs on the low side and 2.2 times the ambient temperature on the high side. You're going to have to look that up.
Pull a 29 inch mecury vaccum and hold it for five hours. If it does not hold, restart vacuum pump and pump back down and maintain for at least 2 hours. The system must bew able to sustain 29 inches without a pump for at least 2 hours. If not there is a leak or fluid left in the system boiling.
The A/C thermometer is about $5.00 at harbor freight.
Remember once you have converted to 134, you have a 134 system. It is not legal to convert a 134 system to R12.
The tag that came with my compressor said 6 to 8 ounces of oil. Another source said 6.8 ounces. Take a two ounce syringe and add 2 ounces to the compressor. Then put the rest in the evaporator, drier, condensor, and hoses. Some sources say 2 ounces in the condenser, some say spread the rest around equally. So 2 ounces in the compressor, 2 ounces in the condenser, an ounce in the dryer, an ounce in the evaporator at the expansion valve, and an ounce in the hoses should be OK.
Always wet the "O" rings with the oil you are going to use before installing.