Good Afternoon I hope someone can help me with this.2010 Ford F550.


so I started hearing a grinding noise on the driver side front. So I figured it was time for brakes which it was so no problems at all replacing them however I'm still having the same noise so I broke it back down and do not see a issue with the brake pads. The calibers seem to be ok . I am attachong a photo of some kind of valve that has a rubber cap on it when i removed the cap i heard alittle air pressure let off is this something that is supppse to add gear oil to? Any help would be greatly appreciated

 

Looks like fitting for vacuum line for auto shuft on the fly 4x4 hubs