I've also been tricked by my own gauge set.Added a holding charge,watched the pressure slowly drop,leak check everything again,etc.The system was fine,the leak was the HP stem packing of my gauge set.By the time I was done ,I had the cleanest compressor in town.An old timer told me "Don't believe everything you see!"Your gauges could read Zero but you could have a full charge sitting on the other side of a check valve.You'll find that out when you open up the system .Lieberts are good for that.