Air pressure drops after moving

Only thing that comes to mind would be a hole in the air line before the glad hand. Hit a bump and the hole is opened, replenish pressure from stopping and the tension created by the pressure in the line has squeezed the hole to a size the pump can maintain the air flow.
Get yourself a spray bottle (Wal Mart or Dollar Store) and spray a soap/water solution on the air line and find the hole.

Since you do drop/hook, it must be a tractor side event, not a trailer issue.

Looking for a pinhole size issue, or a chaffing area possibly close to a securing band.

 

Sounds like air governor going bad
Bleed your air down,see if it starts to build pressure. If not tap on the governor,see what happens.
Could also be obstruction in signal line.

 

or a release valve going bad.
next time chock the tires and go back to the trailer and listen between the tandems if you hear air gushing out, it could be on the tractor or trailer, also if one of the brake chamber diaphragm has a hole in it will do the same as a bad release valve

 

I've had problems with the air lines under the cab getting a rub through at a zip tie, sometimes it would leak. sometimes it wouldn't depending on how the bumps pushed things around

 

few things come to mind, leaking park break can, iv seen them piss out air then leak on the diaphragm gets plugged or it moves far enough that the leak gets covered and slows the leak way down. the other thought is a stuck unload-er valve on the compressor. witch is what i think your problem most likely is. you could take the hose off that runs from your intake to your compressor bleed your air down to kick the compressor in and shoot a half a can or so of carb cleaner down the hose while its building air and then chase it with some wd 40. then you may want to change your air dryer cartridge afterwards.

 

You are going to have to see weather releasing the brakes or not has anything to do with the air loss. If it builds just fine until you release the brakes, then something in that system is leaking. A sticking quick release valve is one thing that will leak big time, then be perfect the next time. If it leaks when the engine is running, and brakes applied, then you are looking at things like fan hub leaking, or if you have an EGR ISX, the turbo actuator could be leaking.

 

This ^^^^^

Listen to heavyD , this guy gives some best advise around period!

 

If you don't find any obvious leaks, a bad governor will do that. Had one that did that, it would work fine then it would stick closed not allowing the truck to re-build air. It's a cheap easy fix if that's the case.

 

I agree with the fellas above. Look at air governor first. Cheap fix and a good knock can possibly tell you if it's bad the next time you have that issue. Then it could be a multitude of items. But fan clutch would be my next suspect.