One thing that might be interesting is to connect your orig. booster externally. By this I mean to take it out of the box, and connect the vacuum hose. I expect you will notice the same vacuum leak. but you might be able to use a chalk puffer or anything that makes smoke to see if you can tell where the vacuum leak is.
I assume it would be some kind of fitting, maybe you can figure out how to plug it when towing.
I can't thin of a reason for a vacuum system to have any kind of built in leak.
This would mean that after a couple of minutes, you would have no vacuum assist if you pressed on the brakes without the engine running.
2014 40QBH Phaeton DP Cummins 380HP ISL, Freightliner XC, 2014 Jeep Trailhawk TOAD
Blue Ox Aventa tow bar, SMI Air Force One -South Orange County Ca.