Fuel return line - should it have any back pressure?

A quick and hopefully easy question…
The fuel return line on the FPR, if I take that off, can I push compressed air through the line to check it/clear it of obstructions? Or is there a check valve that would prevent doing so or risk damaging.

Basically, can I blow the lines free downstream from the regulator?

Bump. Anyone have a opinion?

I believe so since I know of no obstruction to prevent the free flow of fuel down the return line - but I do not see the need for compressed air, surely gravity will do the trick to check if the fuel when under pressure flows freely back to the tank?

That’s the thing that is confusing me.
The scenario: Investigating a rough idle issue.
I’d replaced a noisy pump and filter and still had rough idle so moved to investigate pressure.
I had disconnected the supply side at the fuel rail and ran the pump, emptying into a container. Result: plenty of volume of fuel delivered ( though I don’t have an adapter for exact pressure measurement)
The FPR may or may not be working, I have fluctuating vacuum due to zero compression on #3. But I disconnect at the outlet and run the pump again. Result: Again plenty of fuel volume.

The weird part: FPR is disconnected still, the key is off and I’m fiddling about dumping container into a gas can. And I see that the return line is a pretty strong stream. Strong enough that putting my finger over it results in back pressure that is hard push against. Strong enough and that I didn’t really want to just let it keep dumping fuel, so I reconnected the FPR.
This would make sense if the car was facing downhill, but I have a slight grade to my driveway and this stream is pushing UP against gravity. This is a surprise - should it have back pressure like that? Going uphill?

All this is probably moot, as I later realized my filler drain hole was clogged and the cap was sucking in water from a recent rain. Water in the gas maybe the source of all my problems, but still doesn’t explain the backpressure on the return. Thoughts?