I have attached two photos from Neway of the manual for testing the valve and adjusting. The inspection procedure outline will help you verify the proper function of the valve before ultimately setting the ride height. It is also a good idea to the use soapy water spray to check for leaks on your finished work.
The circled hose is the exhaust line for the leveling valve. There is a slight delay as the suspension goes up and down before the valve actually responds. The easiest way to set dual valves (one on each side of an axle) is to unhook the linkage of both valves and move both leveling valve arms so they are pointing down. That will exhaust the air in both bags. Use the engine to bring the service air up to 120# as noted when the air dryer pops off. Then hook up one side and adjust the bag to its proper ride height. I dump it and reattached the linkage a couple of times to verify I have the right ride height. Then dump the bag you just set, go to the other side and go thru the same adjustment steps. Once the second side is set, hook up both linkage arms. Setting the bags this way eliminates one bag fighting the other when the unit is in operation.
Note that the directions explain how to use the bolt on the arm of the valve for adjustment. Do not adjust by changing the length of the linkage rod. Good luck!