my friends went through this situation last summer. it ended up being the proportion valve. now you say it is original and theirs was not, but there was a problem internally with a washer that was not of the right shape that let it act as a check valve and would hold the brakes (this problem was addmitteed to by the vendors at carlisle last summer). also here is a thread that may help to diagnose.

https://board.moparts.org/ubbthreads/show...;gonew=1#UNREAD