[Fulltiming]

Both the ride height and the solenoid valves can stick but sticking is typically associated with the manifold solenoids. If a down valve sticks, it will exhaust air constantly. Typically, a hide height valve will develop a crack in the housing and leak copious amounts of air, although it may only leak in one position. Not sure why the rear would only have 5 solenoids. All of the ones I have seen had 6, a right up (fill), a right down (exhaust), a left up (fill), a left down (exhaust), and two travel valves that go to the individual height control valves on the rear (the height control actually controls up and down) and on the front both valves feed the single front height control valve. Air from the supply tank to, at least on 1990's models, frame mounted square ping tanks and those feed the manifolds and feed the 3 height control valves to add air to them in travel mode. Sorry, not sure how the tanks are arranged on the Classics. The exhaust when you lower a wheel position is from the height control valve when in travel mode and from the manifold if it is in level mode.

Generally, when you have a sticking solenoid, you will hear air exhausting from the manifold when it shouldn't be. Since you indicate that you can air it up in level mode but when you switch to travel mode the drivers side drops, I would check out the left rear ride height valve first as the suspected culprit.