76 Scamp A/C

You want to make sure you have no hot water flow through the heater core or the A/C will be trying to overcome the heater.

How much R-134A did you charge the system with after changing the compressor? Your low side pressure at 10-15 pounds looks on the low side. Did your new compressor come with a EPR valve installed internally. A true 76 A body compressor would have an EPR valve. Before about 1974 A body cars used a cycling switch after that they went to an EPR valve like the B and C body cars.