Just thinking how the differential vent could be improved. Functions that I can think of:
Prevent vacuum in the differential
Prevent excessive high pressure, while maybe maintaining a slight positive
Keep dirt and debris out
Keep water out, even when completely submerged
Here's what I can think of. Sorry for the Paint drawing, it's all I have available here.
The differential would be connected to the left side of the... box. That system would be isolated from the right side, which is open to atmosphere.
The two sides are equalized at or close to the lowest expected temperature. So long as the differential is hotter than this, there would be slight positive pressure inside. Not enough to damage any seals, but enough to help counter increased pressure when submersed.
The vent to atmosphere would have a simple filter at the end - something that doesn't mind getting wet. If the entire assembly is submersed, water will flow into the right chamber of the box, but no further. This would also increase pressure on the outside of the membrane, further pressurizing the differential while it is submersed.
I don't know much about mechanisms like this. Is it feasible to construct something along these lines that is cheap, simple, durable, and relatively compact? Size would be determined by the air volume inside the differential when it is properly filled with oil, combined with an expected temperature range. Rough estimations put the capacity of the box at 1-2 liters.
One final consideration would be changes in atmospheric pressure due to elevation. Haven't figured out a hassle-free solution to that yet.
Thanks for the thoughts. I know this is overthinking, unnecessary, etc., but I like experimenting with things.