no one way valve. as the fluid heats up, it expands, and would increase the pressure in the axle, hence the vent. as it cools, it contracts, and will lower the pressure in the axle, if it was a one way valve it would suck air into the axle through the seals at the wheels, hence it is just a vent cap, not a valve. you want to extend whatever is there at least to the level of the floor in the truck. as you ford water, the axle will cool quickly, and without a snorkle of some sory, it will suck the water right off the top of the axle is it is under water.