its either the pro-valve or the master cylinder if the fronts are working OK. you would almost have to install some inline brake pressure gauge, try it in the back first to see if the brake pressure is "hanging" after the pedal release, if it is move up to the pro-valve (do GM's have a adjustable pro-valve on the back that senses how much weight is on the back like the old Ram pickups? I know those used to fail alot, thinking the truck was fully loaded down) check it again, if it still hangs move up to the outlet on the master cylinder. something is not releasing the pressure after the pedal release.
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Chris
As close to "Normal" as I can get...
