In the course of owning a 1990 740 for many years, we experienced freon leaks on two occasions. The first time was a fitting associated with the expansion valve just forward of the firewall. Due to constantly condensing moisture on the outside, it corroded through, So we had that replaced along with a new dryer, etc., and converted to 134A at the same time. The second time it sprung a leak in the low pressure line just before it enters the compressor. There's a place where the line loops down and back up, so the condensation on the outside of the line constantly collects there and drips off. Here again, over time, it eventually corroded through the line causing the leak.

It probably wouldn't be too expensive to have an A/C shop put some fluorescent dye in the system and look over the plumbing with a black light. It's a good method to detect leaks. You can also do that yourself but you'd have to buy some stuff.