Originally Posted by PhilipE
Air brake quick release valve.
As much crud as you removed. The air compressor is passing oil. If you don't have a air dryer. I would install one.
Why install an air dryer to fix a worn out air compressor passing oil?
He posted it is directly above the rear axle u joint that was also covered with oil and dirt.
But I would agree as the background would also be covered the same if the pinion seal was slinging oil.
That valve is probably how and why the front of the pumpkin was encrusted.
To the O.P.
Go look at the same dump valve for the front brakes Look up behind the front bumper at the bottom of first cross member of the frame.
If that one is encrusted like the other, your compressor is doing what Phillip says above