Black box with two hoses, what is it

I am a diesel mechanic and I work on a fleet of Isuzu’s, although becoming very familiar with this fleet and the trucks themselves sometimes I scratch my head at things. There is a black box that has two 1” hoses coming out of it, one hose runs to the cylinder head, the other to the bottom of the truck as a what I believe is some sort of overflow relief. This box is located behind the throttle linkage on the left side and directly under the fuel filter and its housing. I could not find anything about this box or what it does. Best I could come up with was some sort of overflow relief. One of the mechanics put way to much oil in the pan when doing a oil change and there was oil coming out of this tube that leads to that box which in return has the other hose that leads to the head. Anyone know what this is?