A/C problem: 1994 Passat

Hi everyone!

The A/C on my 1994 Passat GLX has been losing its charge. I bought a R-134 recharge kit with gauge, and the initial reading was literally zero! So I charged up the system to the appropriate reading on the gauge, and voila, the A/C light came back on and wonderfully cool air came through the vents! Unfortunately, the charge lasted less than 2 weeks

SO, I bought some more R-134 coolant and a can of coolant/sealer with red dye, to see if I could pinpoint a leak. In the process of inspecting what parts of the system I could actually see, I found a large build-up of black gunk on one of the A/C lines running on the passenger side near the air filter box. While I still haven't found any red spots anywhere, I noticed that when I recharged the system this time I could literally see what looked like steam coming out through a sensor attached to the high-side A/C line. I think it was the coolant leaking out. This sensor is in the same viscinity of the black gunk, so I figure it's the source of the leak.

I inspected this part some more, and was able to disassemble some of it. I found 3 different part numbers on it, but haven't been able to find the exact part anywhere (it reads "Ranco SP3-H1027, and 357959139C, and 931603P, and has Volkswagen and Audi logos on a sticker on the side). After doing some research, I THINK it's what's called the "high-low switch," which I've also seen referred to as a "pressure and fan switch." It has four wires coming out of it, and connects to a wiring harness with two red wires and two blue wires. Upon taking it apart, I found that it's basically a cylinder attached to the A/C line, which has a green rubber top that the colored wires go through. But the green rubber part isn't sealed, and if I hold it down with my fingers I can feel the pressure build up underneath. Upon releasing it, I then hear a hissing sound as the gas escapes.

I don't understand how this part works, or is supposed to work. Is it reasonable to think that this is the only problem with the A/C? I'm being anal about it because it looks like the part will be impossible to find used, and is over $100 new!!! (plus labor

). I think this is the part...it looks VERY similar:

http://www2.maximumautoparts.com/parts/ ... category=R

Thanks for your help!!!