Ok, could be a loose hose or a bad brake can.
Is it possible for two of you to listen for an air leak near the two front tires.
One outside to listen and the other to press the peddle with as full a charge of air as it can build.
It is possible the air is leaking past a blown diaphragm.
A cracked air line or fitting.
Bottom line is you need to determine where the air is coming out at.
If you get under the rig please chock the wheels.
2016 F350 6.7L LB CC Reese 28K 2014 Chaparral Lite 266sab
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." 2014 19 days camping 2015 17 days camping201620 days camping