Be sure your test with the water hose is not exceeding the design parameters of the vent. I doubt that the vent gasket is designed to be water tight. Rather, the vent cover is just a loose fitting lid and my guess is that the gasket is to reduce air through the closed vent. That is why the original is sponge material.
Be sure that the space around the vent opening is clean and that there are no obstructions to the vent drain (the hose that points down at the rear of the valve cover in the engine compartment). If you think about it, rain (while driving or standing) would be shed off of the vent cover, around the edges and out the drain.
If you flood it with a hose, I think you will be guaranteed to have water in the cab. Try replicating rain with your hose nozzle (coming down vertically and not too hard) and see if it stays dry. You should have water coming out the drain hose, too.
Kent
