Well, I tend to disagree with everyone but Donnie, although I think David is right about the reed valves causings low high sides along with high low sides. I do not believe that a low charge could cause these conditions in any senario.
I believe overfilling, over oiling, residual flush could cause this or a STUCK OPEN expansion valve. The pressures are not OK. They totally reflect the temperature problem that is the symptom.
Low side pressures on a R12 system should drop below 30psi and the system should cycle when achieving its goal. The temp of the evap will be almost a direct conversion from the pressure into deg F. In other words something like this (I do not have the chart handy) 30psi = 35deg F, 40psi = 45deg F, 50psi = 55 deg F.
I would replace the exp valve and drier and evaluate the amount of oil that is released on evacuation. I would also see how much oil was in the drier and/or cut it open for a view of the inside of the system.
Absolute temperatures or pressures are hard to specify as this is a dynamic system very much dependant on the ambient temperature. If the day was 100 degrees I would accept a duct temp of 60 degrees and the resulting pressures at 50mph and hiblow. At idle they would go to 70 deg and probably 60psi.
Actually I would accept those numbers for a 134 conversion but it would be very borderline for a R12 system. A real good system would knock 50 degrees off ambient at 50mph hiblow. Most new 134 systems easily do this.
Bosch Master, ASE Master, L1
33 years MB technician