Brake Question
 

Working on 96 Chevy 1500 4x4 Suburban rear brake's locking up,Trying to help out a friend, removed drum's everything looks good :!: :!:

are they locking up hot or cold? after a short or long distance? do they release after it comes to a full stop? lots of scenarios here from bad proportion valves to sticky master cylinders. with drum's though it shouldnt take long to figure after a couple of different checks and tests...

They will lockup anytime, will do it in garage,driveway, hot or cold, only rear doing this, The guy i am helping don't have alot of money so i have got to be right on this one, they will release normal. Not sticking at all just locking up before front does. shoes looks good wheel cylinders good, no axle leaks. Just a quick lockup.

front part of master cylinder could be failing. porportioning valve failing,

how is the condition of the brake fluid? colored, contaminated, milky, etc?

Removed all fluid and put new in, bleed out still same problem.

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.

no it doesn't have the valve.

i would take alook at the emerg brake cable, could be frayed and not released all the way, good luck.

I would look for a flexible brake line that has been damaged and swells up. It will allow pressure to be trapped and drag the brakes.