Mike did you put on two tap a lines? for high and low readings did you do a vacuum on system did you change out the dryer? if the pressures to no equlise on both side this may be why compressor is locked up when you seal cap line did you cold it up perverting pressure changes? When I measure Freon I use a special cylinder it read oz on side when you turn tank upside down turns to a liquid as it fills tube you read the scale on side so this is my scale so of the way school showed us. Many company's will not consider using tap a line because of movement of and RV's in general and vibration most say toss it.