Chevrolet Repair: 1997 Express Van G1500 weak rear brakes, rear brakes, rear shoes

Question
I have noticed over time that the rear brakes feel as if they are weak. I have replaced the rear cylinders, drums, shoes (GM oem shoes), master cylinder, adjusted, etc. I occasionally tow a two axle trailer with electric brakes. When braking hard I know the trailer brakes are working and under hard braking I can invoke the ABS on the front. It feels as if there is practically no braking action on the rear brakes. When I installed the rear shoes they were correctly installed and adjusted. I have tried different adjustments but no help. Suggestions? If this system had a conventional proportioning valve I would replace it to see if that helped.

Answer
Ther probably is not anything wrong with the rear brakes.  On that van the fronts do better than 80% of the effort involved in stopping the van.  It is not uncommon for the rear shoes to last over 100,000 miles while you replace the fronts every 30,000