Axle vent/breather

[400ex]

After finding what looked like chocolate milk in my rear axle, I had the thing rebuilt only to notice that there was no vent or breather on the end of the hose connected to the axle housing.

I am trying to locate a good vent or breather to connect to the hose. But my question is about the vent itself. Is it a one way valve to let pressure out but not suck back in? Or is it just a regular air flows in both directions type.

What have you guys been using if yours needed replacement?
Suggest away.....

Thanks,

James