What is this valve?

[miniteeg]

It is connected to a hose but I failed to find its function on the owner's manual.

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[crash68]

It's definitely aftermarket, almost looks like a fitting for compressed air. Maybe the previous owner had onboard air compressor with a tank. Follow the tube see where it goes.

 

[Blackdodge2010]

It's definitely aftermarket, almost looks like a fitting for compressed air. Maybe the previous owner had onboard air compressor with a tank. Follow the tube see where it goes.

This.

 

[Lee Peterson]

Check in the back shock springs someone probably added air bags to help sag when towing.

 

[werdna1977]

^^^i agree