The evaporator is supposed to be at around 40 degrees temp at design conditions. That is 37 psi in the evap and 157 psi in the condensor or high side R-12 at design conditions of 95 degree ambient. You car uses an expansion device instead of a fixed metering device. When the car is off the pressure in the evap will rise slightly closing the expansion valve and sealing the evaporator to a degree. That is why you loose less refrigerant when the car is off.
Here is some A/C help for your car.
http://dieselgiant.com/repairyourac.htm