I have a 91 Volvo 240 station wagon with 110,000 miles. The A/C was converted to 134A about 8 years ago because the A/C shop I used would not do R12 work(I now regret that decision to convert). I want to add a bit of refrigerant as it now only cools down to 60 degrees F. I want to be sure I add at the low side (suction) port. As I look at the setup on the right(passenger side) side under the hood near the firewall, I see a 134A-type port. It has a blue cap screwed in. Nearby is a black tall "cap" that appears to be something one could screw off. I tried with a wrench, but it would not budge under mild force.
My question - How do I identify the low side port?
Observation - While working on the front of the engine a couple of years ago, I noticed what appeared to be a port on a line running along near the bottom of the radiator. I guessed this was the high side port, hidden away so that those of us who do not know what we are doing cannot accidentally use this port to attempt to add refrigerant. Would this be a correct observation?
Second question - What is the black "cap" that I see?
Third question - How do I load a picture to this question so readers can know to what I refer?