Even when they were new, these systems were never good about holding a charge for long periods of time - especially if the A/C isn't used often. Your best bet is to put dye in the system, jumper the low pressure switch, and look for your leaks. Good news is that the price of r134a is now starting to come in line with what Freon used to cost in the '80s, a couple of bucks per pound.

The A/C relay just fires up the blower motor (if it isn't already on) when you turn on the A/C.