I don't see any rationale for using FR-12. Since it's mostly R134a, it cools just like R134a (not so good...). Most shops won't service your car - they won't touch these blends. When it leaks, the different fractions leak at different rates, affecting the performance. Guess what I'm getting at is you should just perform a 134a conversion if you have to go into the system anyway.

On my 1984 300D the high side fitting was in the metal portion of the high side hose before it entered the condensor. As I recall, it snaked around the botton of the compressor, then the front of the oil pan, then up and over to the condensor. The high side fitting was kinda sorta on the passenger side of the oil pan. You had the crawl partway under the car to access it.

Seeing zero PSI on the low side with the compressor running indicates a severe undercharge - you have a leak somewhere. That the compressor runs at all indicates the cutoff switch - mounted on the receiver - is seeing at least 2 bar of pressure, so there is some refrigerant in the system.

Good luck.