Rear A/C Leak

I have a refrigerant leak in one of the lines that goes to the rear unit but I don't know exactly where because the transmission is blocking the location. Can I service these lines from the top with the doghouse removed? The van was converted to R134 by someone else. I see near the rear AC there are service ports. Why are they there? Doesn't the whole system run on a single refrigerant circuit? When I recharged from the main ports under the hood, the rear AC worked fine except for the leak, so I don't understand why there are separate ports in the back. Also, these ports look like they are still R12 fittings, so they weren't retrofitted but I don't know why?