Andy, your Bosch in-line IP (and injectors) have a return line.
But the return system isn't your problem. (As it can be with a Stanadyne rotary pump).
You have a restriction between the tank outlet and the electric lift pump, or a bad lift pump.
There is NO way for air to get into that fuel system, once it is bled, while the engine is running if there is fuel available to the electric lift pump and it is functioning.
The tank outlet is below the level of the fuel, as is the electric lift pump.
Any leak there and you'd have fuel dripping on the ground.
Once the fuel gets to a properly functioning electric lift pump, it is forced upward to the transfer pump on the Bosch. And any leak between the electric pump and the Bosch will spew fuel under 6 to 8 PSI of pressure.
If you want to get serious about this, rig up a "T" fitting at the input connection to the transfer pump on the Bosch IP and monitor fuel pressure.
I don't have factory specs in front of me, but you want to see positive pressure, even when the engine is "dying" and 6 to 8 PSI is an educated GUESS.
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