When I start the car for the first time and put on the A/C all is fine. However, when I restart it later, air coming out of the middle vents are warm and the air out of the defrost and foot vents are warm. The A/C system is fully charged and the compressor is working but not cycling on and off, just stays on. The low pressure tube is cold to the touch. I was wondering if it could be the heater valves? Any help would be greatly appreciated.