Weird air system issue

[KF6-BusFan]

So I have to assume the issue is inside the drum somewhere?  Any advice on what could be keeping the rod from disengaging? 

Did you get to the bottom of this?  The DD3 air chamber needs air supplied to what they call the "locking port" in order to unlock the brakes. This comes from the inversion valve, which is commanded by your push/pull parking valve.

The locking mechanism inside the air chamber is a sort of ratchet, so sometimes after applying "lock" air to unlock the lock, you have to pulse the main brake application air, to push the rod out briefly and then allow it to retract.

While working on mine, on the bench, I would test by applying air from a compressor/tank to the lock port, and every time the rod would just retract using the internal spring. But when installed, sometimes I have to pulse the brake pedal.

Hope that helps.