1994 Buick Regal Recharge coolant

The low pressure port on the above car is located ON the low pressure ( the thick ) line that is going to the IN side of the accumulator. The out side line/hose of the accumulator goes to the compressor
It is a lot easier to put your service gauges onto this port if you first remove the air cleaner assembly and the port is then very easily accessible.
But if you have a Regal with the 3.1L motor then the Low pressure port could be iether as above or it may be located on the accumulator proper, most likely adjacent to the low pressure switch.