Have very little pressure at rear brakes when bleeding. New master cylinder ( bench bled) and wheel cylinder. Unhooked brake line at proportioning valve and get fluid but not strong stream. Can proportioning valve be partially block or only slightly working? I am wondering if it is a part that either works or doesn't work?
Do you have a height-sensing proportioning valve like most trucks and minivans have? If so, there won't be much fluid flow to the rear if the truck is jacked up and supported with jack stands under the frame and the axle is hanging down.