I don't know if this is your problem or not, but I had a similar issue with my truck where I had very little brakes on the rear. On my truck with drums,there is a load leverage bar that mounts between the rear-axle shock mount and above on the frame is a load sensing type device. When a load is placed in the bed or a trailer coupled to the hitch,the rear suspension squats and this lever moves up... . which in turn,allows more hydraulic pressure to the brakes in the rear drums. I had a problem with this and some here suggested taking this out of the brake line circuit. I chose not to do this... . but what i did do was to add a 2" steel shaft to the length of that rod and now I have brakes lthat actually work!!I don't know all the trade names for the parts,but my description should allow you to see what I'm talking about. To prove or confirm this the faulty item... simply remove the bar from it lower shock mount and temporarily raise it and secure it higher to simulate a load in the bed. Go for a test drive and I'll bet you'll have better brakes. Something to try!!!!
Alan