回复：low pressure side of what?

The AC line has two side: low pressure and high pressure. The line from the orifice to compressor section is low pressure side, and between compressor and orifice is high pressure side. There is a port at low pressure side that fits the fitting on the gauge. That port is also for filling refrigrant purpose. You should follow the two lines in and out the compressor and the line with filling port is the one we are talking about.