I'm new here, and excuse me for getting in, I just got my trike (never rode one before) not even two months ago, so I'm begining to learn about it.
Mine is a GL1500 '95 "A" and has three air shocks in rear (one OEM plus two similars) and all of them hooked and commanded by the OEM compresor, the control panel and display working Ok.
I guess there are 2 different air circuits, that the output thru the spare hose is straigth from the comp and not controled at the display, you'll probably need to connect to the original port going to the air shock. Just a thougth!
Mario