There have been situations when you go to work on a refrigeration unit that is acting strange as far as pressures and either wont work right or you just cant make work properly. The question is is it overcharged, is there air or non condensables in the system or is there any restrictions in the system. After making sure that both coils are clean and the evaporator fan and condenser fan are running I have found that is best to recover the refrigerant, evacuate the system and weight the charge in with a digital scale. Now you know that the system is properly charged. also before any of this is done it is a good idea to check and make sure the evaporator coil is not iced-up. A frozen coil can give you all kinds of weird readings.