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You can sometimes get a flap of material that comes loose inside the flex line that will "flap" over the opening and restrict fluid flow. Your proportioning valve could be the culprit though. Have you tried pushing the reset button at the front of the valve? There should be a rubber bulb at one end of the proportioning valve. If the system becomes unbalanced and the valve travels to one side, that button lets you push the valve back into position. Sometimes the valve can be difficult to get reset and you will need a tool to help you move it.Here is a decent troubleshooting guide: https://www.classicperform.com/TechBook/BrakeTroubleshoot.htm#testprop
Getting closer...
Stock combination valves are fine. The more you deviate from the factory setup the more Frankenstein the truck becomes. The factory braking system will lock the rear brakes easily in a hard stop. You've definitely missed something!
Bypass the combination valve and see what happens You could also have a pinched line between there and the back axle If you want to find out where the problem is You need to get a gauge and start testing it at every junction starting at the wheel cylinders