If you fill a bulk tanker to 12-13 or whatever value below limits, and kept the product discharge line off and all pots closed when empty walk around quickly listen for leaks. HOPEFULLY its not up top in the hatches. (DO NOT whatever you do go up there.)
You mentioned unloading with all pots open. Erm I don’t do that. The only time I have all pots open is when pot one and two are empty and the third is just about down to empty, shes singing at this point and there isnt much left going up to silo anyway being essentially empty. Pressures do fall across the board and if you leave it hooked to say a export silo complex you will start to parasite air away from the other 20 tankers unloading off plant air and plug all of them up. Might cost you a gallon of blood.
What I do generally is fill a tank to 14 or so. Open one pot. Unload that pot. Line pressure should be moving product to silo briskly. IF there is a leak somewhere in the discharge line it’s going to show itself pretty well which leads me to the original thought of bad seals up top which is possible. That is what the 5 gallon bucket of water is for (In cement) vs discharge line leaks.
You also have a screw driver of both types there are those rings with bolts on them that work loose now and then. Thats where the majority of the leaks happen.
Once that pot is empty, it’s closed. Air built back to 14. Open middle pot. Repeat. Empty, close and blow the discharge line good clean air all the way to silo. (Check top of silo for excessive dust particularly in small ones in precast) build tank pressure up to 14, reopen discharge line then last pot. Empty it.
Once that pot is empty and discharge line is empty, leave last pot open, open number two tank pressure should start falling already, then open last pot pull the tank all the way down as you can. If its singing alls well. And you know you are empty. (They really hate it when you leave product unloaded) This last is a bit excessive for some but its a routine I follow as a check against leaving product in tank.
Dump tank air via emergency discharge line after checking to be sure no souls are walking on that side near that outlet.
When everything is zero on gauges you are ready to go after wrapping hoses etc.
The only picky thought I have here with OP is running all three pots at once. It’s not what I do and I don’t think you can support tank pressure feeding all three pots creating a risk for plugging somewhere. On plant air maybe. But not on blower exhaust air.