To figure out if the compressor is working, you have to actually watch the compressor clutch to see if it engages. There are several reasons why it might not: Low charge, bad clutch, bad compressor, bad ground. Until you establish whether or not it's actually spinning, you don't know if the comp even works at all.

By jumping the pressure switch on the receiver-dryer, you can establish the compressor function. If it doesn't go on, there's a problem with either the wiring or the compressor.

The tarry goo over the pipes and the expansion valve is standard equipment. Nobody was trying to fix a leak, probably. The typical condensation leaks are from the bottom drain of the evaporator, on the right side. It goes straight into the transmission tunnel. You can spot it easily--the thing gets crushed and collapses. It's a soft, rubber tube.

If you're planning to replace the compressor (if it's bad) you might want to consider doing an R134 conversion. The 134 is not as good of a refrigerant, and you'd need to change out all of the O-rings in the system, possibly the condenser, and the expansion valve for best results. If you put R12 in an unknown system, price can start to escalate out of control simply with the price of refrigerant being what it is.

--
chris herbst, five volvos.