Losing air while bobtail and detached from trailer.

Trailer air supply line

 

Tractor protection valve?

 

you got a leaking dry rotted air bag. the weight of the trailer pushes down air bag. look and spray where the folded part is. top and bottom. when your bobtailing around. the bag is taller. exposing small cracks

 

Levelling valve on suspension ..

 

Ummmmm, air bags run the same height all the time, that's why there's a leveling valve?

I second the tractor protection valve as the culprit. Anything air suspension related will be worse hooked up due to higher weight / pressure.

 

They do ride at slightly different heights depending on weight. Its not a big difference maybe a half inch or so, leveling valves aren't that precise.

 

so i guess you never had a dry rotted cab or rear axle bag lose air.

 

Plenty. And always worse with higher pressures.

 

It's not the weight, it depends more on where the suspension happens to be within the very small dead zone in the center of the leveling valve and/or slop in the linkage. They are a lot more precise than you think, it's much more common to see a loose arm causing varying ride height than the valve itself. I run a carhauler, changes in ride height are quite obvious as low to the ground as I am, and it's remarkably consistent.