It's always helpful for us to know the year and model that you are working on.

From your description, it SOUNDS like you're working on a 124 car. That said, oil around the seem of the expansion valve could be a clue, but not enough evidence to know for sure that this is where your leak really is.

Given that this is an R12 system, according to your comment about adding R12, AND the fact that it may have lost a can in only a few weeks, you don't want to look for the leak with dye because you would lose the necessary R12 to the atmosphere. This is, of course, harmful to the atmosphere PLUS the stuff is not cheap.

I would suggest jumping the low pressure switch to ensure that the compressor will kick in. Only leave it jumped long enough to varify that the clutch will kick in. Once you do that, then you can consider that the electrical circuits are probably okay.

Once you have determined that, I would suggest that you find a shop to look for the leak. They have several ways of doing this that will be less expensive than losing a charge of R12. Once they tell you what component is leaking, you can then replace the component yourself and then take it back to the shop for them to evacuate and charge unless you have a vacuum pump and manifold gauges to use for this job.

Good luck,