The exact location of the low-pressure port varies between different car models. But you can easily find it on any vehicle in just a couple of simple steps:
Start at the A/C compressor. It's attached to the front of the engine, and is powered by a belt and pulley. It can be either at the top or down along the side of the engine, as shown in this example:
One way to tell the compressor apart from
other belt-driven parts, such as the alternator, is to turn the engine on but turn the A/C off; the compressor will then be the only pulley that's not turning. The the A/C compressor can also be identified because it has two metal tubes coming out of it.
The two tubes are different sizes. The smaller tube is the high-pressure line. The larger-diameter tube is the low-pressure line, which is the one you're looking for. It runs from the compressor back towards the passenger compartment. Follow that line until you find the one and only valve that sticks out of it, which looks like this:
It should be covered with a plastic cap, which you can unscrew to access the service port.