hello i have a problem in my 2006 econoline turbo diesel dobble ac unit is an ambulance, when i start running my van the ac is working great for about 20 minutes after that the blower and the compresor still working but in the front cabin is not cold but the rear unit still working i have check the gas presure and it have the correct preasure 45 240 psi if anybody can help me would apreciated thanks