There is no backpressure. From what would there be any pressure? The air in the hose is the exact same pressure as the water in the tube and the water around you. There could only be backpressure when there is a difference in pressure anywhere in the system but there isn't.Wouldn't the backpressure experienced when trying to pee through an unbalanced p-valve that you did not prime depend in part on the depth at which you first try to pee? By not priming, the tubing is left full of air.
Not really, the H has a check valve under the knob, right? Did you take out that check valve too?Voila--conversion to unbalanced p-valve. That did the trick in stopping the leaking. I do not have an opinion on any potential "squeeze" with the resulting unbalanced p-valve because I always primed it before descending (almost always need to, thanks to the coffee I drank that morning). I didn't do any experimentation with not priming, descending, and then trying to pee at depth.
The check valve is only in there because of the duckbill. Maybe you're thinking of backpressure because you only half-modified your valve. A normal unbalanced valve has no check valves. It's just a hose you can plug at the end.
When you just plug the hole were the duckbill was you have to prime and do run the risk of a squeeze. Take out the check valve too and you don't have to pee in the parking lot or on the surface.