Hey did you ever find the issue? I know this was a while back. My friend has the same issue , I work on cars as a side job. I flushed the system 2 large bottles of brake fluid, took apart the master cylinder, cleaned everything, same issue the brakes lock down slowly as the motor runs. It will release the brakes after I disconnect the vacuum line to the brake booster and press the brake peddle. I have read the brake booster can do that if a valve inside is malfunctioned.

 

I would like to know too as I am having the same issue with my 2006 Tahoe...