I suspect you will still find leakage elsewhere. There is an output check valve on the air dryer, so once the air is pumped past that, there should be no going back to the compressor... If it is happening while driving, i would first inspect the park air lines, anti-compound valve and spring chambers. a park chamber leak would pass out thru the cage bolt holes. Depending on your year, you may also have a remote control valve inboard of the pri-sec tanks that disconnects air to the air/hydraulic pump when park air is applied.
Chock the wheels release the park brakes, a bad 2way could send park air out the front red glad-hand vent. Are the rear glad-hands capped/sealed? Fronts are vented, rears must be sealed...