C 30 brakes

[jeepoguy]

Hi all new guy here. I have a question on rear brakes for my 1983 Chevy 1 ton dully. There is a brake valve mounted on the frame with linkage running to the rear axle. Now I understand that this is a load sensing device that controls the amount of braking to the rear wheels. The one on my truck has not worked for a long time. With the bed loaded and my trailer with my jeep on it you can stomp on the brakes and the front wheels and the trailer brakes will lock up but rear wheels on truck nothing. Any thoughts? And yes all the component such as the shoes and wheel cylinders are good.