Which vaule is low-side service port?

Question

I have a 91 Ford that the air conditioner works. However, I need to add coolant. I need to convert the R-12 to R-134a. Which vaule is the low-side service port and which is the high. I do not have ta service manual.

Answer 1

the small line is high pressure and the big line is low.

Answer 2

As the previous answer states, small is high, big is low. I just wanted to add that you should slip in a new orifice tube when changing the system over. If you are using a kit designed to retrofit a Ford, then it should have one. If you just using a universal kit, you should get one from the dealer or from your local A/C shop.