The clutch does not cycle as often on a variable displacement compressor. It more or less stays engaged the entire time AC is selected. When there is less demand, the compressor reduces it's output by shortening the stroke length of the compressor. This a computer controlled system which anticipates demand and varies the position of the solenoid or spool valve within the compressor. The valve uses system refrigerant pressure within the pump/compressor to alter the angle of the swash plate, hence lengthening or shortening the stroke of the pistons. The solenoid/spool valve has been known fail. Unfortunately replacing this valve entails draining the system of all refrigerant.
It cannot be said what the problem is with your car until more diagnosis is performed. To properly diagnose the system a good scan tool and gauges are required and someone who knows how to use them.
Refrigerant travels in a continuous loop motivated to do so by the compressor/pump. Half way around this loop is a small orifice which restricts the flow of refrigerant but never totally blocks it. As the system sits inoperative, refrigerant gradually flows through the small hole [orifice] equalizing pressure throughout the system. Differential pressure between the high and low side only exists when the compressor is in operation, and for a small amount of time thereafter.
Usually the problem with an older car is the system has lost some of the refrigerant. If you can get the system operating and post what information you can, some one here might be able to help. Again, hot and cold pipes without compressor operation does not make sense. A re-test is needed.