Need help determining what this is for.

[reelclassic]

I have an oil leak and cant figure out where its coming from. The engine itself looks pretty good. I have attached a photo of what i think may cause it. I have two of these
on my truck under engine - driver's side and passenger side. It looks like it has a fitting where a hose should attach. Any ideas?
Thanks for your help.

 

[Suprman]

What truck?

 

[reelclassic]

Superman its a 2003 LMTV 3126 Caterpillar truck. I have (2) of these connected to the frame and it looks like the black fitting should have a hose connected to it.

 

[scottmandu]

That's the ABS air valves. I believe there is one for each wheel. The black cap is a vent to atmosphere.