one thing that will always cause high discharge pressure is air and non condensables in your system. those can only be removed with a vacuum pump, being an automotive ac has a very small capacity, it dosn't take very much to cause this. if your suction pressure is low, it is likely a plugged orafice.
in a small refrigeration system, one cant over stress making sure the system is free of non condensables, and also be sure to weigh in the correct amount of refrigerent as the vehicle specs.

Bill