"Normal" pressures depend on ambient air temperature, and humidity. The hotter and more humid the air is, the less efficient the cooling will be. R-12 usually reaches 32 degrees F. at around 28-30 PSI suction pressure. Most systems were designed to cycle the compressor off just as the evaporator coils would begin to ice over, then kick back on at around 35-45 PSI. If the compressor failed to cycle, the evaporator would freeze over. Watch the suction pressure as the compressor trips off- - - - -if it gets below 20-25 PSI, something's wrong with the clutch control circuit. I started doing automotive A/C work in the early 1960's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!