The ac compressor recycles on and off at 2-3 second intervals, and does cool, but not like it should. I tried to charge with refrigerant, but when the engine is running, the low side port is pressurized... it will not take any charge. Is this a sign that the refrigerant is not looping thru the system? Could this mean a plugged filter, or a check valve isn't working properly? Any suggestions....

 

There are no check valves in the system. The orifice tube or receiver could be clogged. You need gauges to more accurately diagnose your system. To ensure the lowest pressure in the low side, turn your AC on max with outside air , the temp. slide on cold and the fan in high. Do all of this for at least 5 minutes. Then attach the can. Remember that the low pressure switch controls the length of cycling. If it fail (fairly common) it will also give the indications you describe.

 

Thanks much Missleman 740! Will do.