Low side refrigerant pressure should be measured at the valve with the black cap behind the A/C compressor. It's kind of hard to picture right now but a metal tube sticks straight up with a larger metal fitting and a black screw-on cap. It looks like a large ball bearing is trapped in the hole in the center. Get a refill kit and read the instructions. If the freon is low on the guage, fill with 134a wile A/C is on high.
