the filter screen on the orifice tube could be dirty/ its inside the high pressure pipe going to the evaporator/ parts cost aprox $12 and evac & recharge the system. unfortuneatly there is no high pressure fitting to check the high or liquid side of the a/c, if threr was, the pressure would be over 250-300 psi when the system returns to neutral pressures it will partially clear the screen . note: the orifice is the same as a G.M. car part(chevy ponti, etc.)in the same year range. note if car is losing freon most probable cause is rust pin holes in the reciver/drier unit