Proportioning valve question

[black_k5]

Hey guys I have a question on my brakes. Last year I swapped my axles for a set of 3/4 tons out of a 86 suburban. These have the optional larger rear drums, I think 13". When I brake my brakes light comes on and I have poor braking since I assume the piston shifts and cuts off the pressure off to the rear. I was told that the new wheel cylinders are larger and take more fluid which causes this. They suggested adding an adjustable restriction to the rear line but I'm not sure that will work with out removing the factory prop valve and replacing it with a distribution block. The other option it to go back to the 1/2 ton axles. Is there much advantage to keeping the 14bolt semi float? Thanks guys