porportoning valve ratio change

[77leo79]

I have a 77 & 79 Heavy Half Factory stock, one thing I need to figure out is how to put more load to the rear brakes. when loaded on damp or wet roads the front locks up too quickly. All adjusters are in perfect working order,rear drums,pads, new master cylinder,pistons and calipers are excellent. I am a firm believer in using the valve, so is there an adjustment or fix-it or just replace it? I am an ex-18 wheeler so I have a lot of driving experience and am fair to good mechanic with the tools to play with this issue. Any advise on this subject will be appreciated.