R-134a can work reasonably well in the 124 chassis if all the components of the system are in good condition. I used in my '87 after replacing the evap, condenser, and compressor. It handled Dallas heat without too much trouble. Of course, curiosity got the best of me so I evacuated and went to R-12 (after determining the system was leak free...). And R-12 does work better - faster cool down and slightly lower vent temperatures in the end.

Back to the original poster's question. I actually don't know whether the valve on the compressor is high side or low side, never messed with it. The valve on the line up by the strut tower is indeed the low side. High side valve is down by the fan belt, near the compressor pulley.

You will most assuredly get very poor results if you release all the R-12 and then charge with the death kit. The a/c system must have only refrigerant inside it - no air, and no water vapor. To achieve this, an extremely deep vacuum is pulled on the system prior to charging. To put it bluntly, if you let air into the system, you're screwed. It won't be cold, and it will be damaged.

In the U.S. it is illegal to add R-134a to a system containing R-12. Don't know what the law is in Canada.

- JimY