Cusser
+1y
I would contact the experts at AMA auto refrigeration (ackits.com) and ask them. My opinion is that it would likely be OK, flare fittings have been used for refrigerants. Propane is generally NOT at high pressures, because its at equilibrium with the liquid propane.
To just connect tubing, here in the lab even for small flammable and explosive gases like hydrogen, we use SwageLok connectors. They seal real well, and are available in standard and metric diameters, and in various metals to match up with the tube metal.