That's just a simple air vent hose from the vent on top of the axle tube, there's no valve. Hard to say where that stain is coming from, clean it off with Simple Green and wait for it to start reappearing again. You should be able to tell by watching it after it has been cleaned. If you would just follow that hose, you'd quickly and easily find its other end is inside the rear tub corner next to the gas fill inlet.
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