[kipps]

I don't know much about newer common rail systems, but I've been around plenty of agricultural mechanical injection diesels. Most returned to the top of the tank. I assume it was simpler, with one less connection to leak.

Smaller Case skid loaders dumped the return half way up the side of the fuel tank. If the tank was more than half full, the return line would leak if disconnected. The large frame Ford tractors, almost from the very get-go in the 60's, used two tanks. The top tank merely gravity drained into the lower one. All fuel line connections were in the lower tank, which meant the return was "under-water" for half of the time. All seemed to work fine.

I don't know how air bubbles would be getting in the return line in the first place. The fuel coming back from the engine should be perfectly clear of air.

The return line doesn't like to be blocked, and will keep an engine from running if it is, but based on the older engines at least, it will build tremendous pressure if blocked. A few times I forgot to remove the vice-grip from the return line and blew the connections apart upon starting an engine.