Your line thats leaking is the one that is COMMING FROM the side of the ride height valve in that rear corner as the photo Gill posted. Not to or from the front. There should be 2 valves there side by side. Whatever valve it is attatched to thats the corrisponding rear corner the line comes from.
With the correct supports in place the tires do not even need to be removed thats less than an hours work and about 20 bucks in plastic 3/8 line and fittings. The fitting at the ride hwight valve end is 1/4 pipe to 3/8 compression. The fitting at the valve is 3/8 pipe to 3/8 compression
The other line coming IN the bottom of that norgren spool valve is COMING from the norgrin air manifold under the drivers area it produces air or lets it out when u hit the up and down button at the drivers seat or when Iin auto level. The top line is GOING to the air bag.
The 2 smaller black 1/4 in lines on the ends r the signal lines. One is energized when in driving mode the other is energized when u r in level low manually setting height or when the cc auto level is turned on.
When u turn the key off neither signal is present. If the valve is completly air tight u can pull every line out of that valve accept the top one going to the air bag and that valve should maintain pressure to that perticular corner of the suspension. Thats actually the way to bench test those norgrens right on the bus.