The low-pressure port is usually smaller than the high-pressure port and will be fitted with a blue cap. Look for a fitting with a blue cap on the accumulator/receiver-dryer.
However, there's no guarantee of placement. Retrofits are often done by different mechanics using different parts and strategies. While the accumulator is the most common location, it's possible the low-pressure port might be on a different part of the A/C system depending on the specific retrofit kit used.
To be absolutely certain:
1. Check the retrofit sticker: It *might* indicate the location of the service ports, though this is not always the case.
2. Consult a service manual: A repair manual specific to your 1990 Buick Regal will show the A/C system's layout and service port locations (even for the retrofitted system).
3. Look carefully at all the fittings: Trace the A/C lines from the compressor and condenser, paying close attention to any fittings with colored caps (blue for low, red for high).
If you're not comfortable working with refrigerants, it's best to leave this to a qualified professional. Improper handling of refrigerant can be dangerous.
