I wouldn't replace any O-rings that you don't have to. Disconnecting an otherwise good line to replace a seal may destroy the line in the process. The fittings become corroded and the nut can shear off the end of the tubing, leaving you with a very expensive problem. Anyway, as said, the high pressure liquid line is the small one that runs up the frame rail on the passenger side. The line connects to the evaporator inlet at the firewall on one end, and at the condensor outlet at the other end (the connection is under the car where it cannot be seen). There is also a large diameter line right next to it that connects to the accumulator (the giant aluminum cylinder attached to the strut tower/firewall). The one that usually corrodes is the small, high pressure line. As Chris had said, a steel clamp on an aluminum line is just asking for trouble (why does Volvo do that anyway?). The dissimilarity between the two metals usually causes corrosion at these clamps and then the line needs to be replaced (that is exactly what happened to the line on my '88 760). If you do replace the line, I would suggest cleaning the clamps thoroughly and then applying a light coating of ATF between the clamp and the line. The corrosion inhibitors in the transmission fluid should help to keep the problem from recurring. Anyway, I hope this helps...