Thanks for the replies. We are, in fact, replacing the receiver/drier. The compressor seems fine, so unless we find leaks, I see no reason to replace it. We'll vac it down thoroughly, add oil and R-12 (the R134a is less efficient in a system designed for R-12, and this system is marginal even when in good order) and see how it cools. I have a gut feeling it'll work fine, and I'll let y'all know how it turns out.