How to add R134a to my van on 1993 Chevrolet G Series Van (G20)

How do I identify the low pressure port ? Any other tips would be greatly appreciated. The a/c works, it's just getting weak. It has probably been 6-7 years since it was serviced last.

If your using a can of refrigerant from a auto parts store the fitting is designed to only fit the low pressure port, be careful not to overfill.
here's a site that has instructions for recharging your system the fitting on the can only fits the low port
The auto store recharge kit did not fit the nipple with black cap. There is another one sticking out of the evaporator ??? canister that has no cap. Totally uncovered. Might that be it ???