Auto Air Conditioning & Heater Repair: Pressure Reading, grand caravan, 90f

Question
I have 94 Grand Caravan. NYC temp is around 86F now. AC is working I guess (Temp dropped from 90F to 50F in less than 5 during highway driving). Temp reading sometimes is less than 40F in the city driving. Howerver, when I measure the pressure on LO/HI. Lo reading is staying around 40psi, but Hi first read 140psi then dropping slowly to less than 100psi (as low as 65psi). It this pressure reading normal? I don't know if I have enough refrigerant. Should I add more? How much more should I add?

Answer
Hi Woodman

Need to know what your when your high drops does your low drop with it.  If it does then your expansion valve and drier must be replaced.  If it does not try another gauge and let me know.
With your low at 40 psi you should have a higher head pressure.  That's why you should check it with another set of gauges.