Rear brakes locking up
 

have a 2007 Chevy express 3500 passenger van. 100000# towing capacity. I think it is called a brake proportioning valve or metering valve to help direct pressure to the back brakes. It is located on the driver side frame. It has an electrical connection with one brake line and on hose connected to it. Looking to see what the correct name is so i can get a replacement one.
My brakes on the back just started to intermittently lock up whether towing or not. replaced both rear calipers. bleed brakes put on new brake booster but has not fixed the problem. any suggestions. thanks.

Wow 50 tons! You could tow a 737 with it. :p

You are thinking of the Brake Pressure Modulator Valve (BPMV), which is part of the ABS system that controls the entire braking system. It's attached to the ABS pump and EBCM on the frame.

Dyanamic Rear Proportioning (DRP) is controlled entirely by the ABS system. No dedicated valves.

When you service the hydraulic brake system, it's necessary to perform an ABS bleed with a diagnostic tool to purge the air.
Failed flex lines can also cause brakes to drag.
Or possibly a malfunctioning parking brake.