Adam I want to be sure you are not hearing air exhausting out the short tube that just tails off hooking to nothing. Put your finger over that and see if it stops. If so at worst your ride height valve may be getting slow but working as designed. As the bus gets closer to ride height, coming down, the airflow decreases and some ride height valves take a pretty long time before the exhaust port completly seals off. It is like the closer the bus gets to height the smaller the leak becomes., finally it stops
I am probably way off base here
How do you know it comes back when you are rolling down the road?
Sorry for the spelling folks my hard drive puked I am on a borrowed laptop and can't find the spell check.