I came across this nice proportioning valve today that might be of interest to you guys doing the 4 wheel disc conversions. It has a built in adjustment knob. They are a good price also. About 1/2 the price of an aftermarket fixed proportioning valve.
http://www.summitracing.com/search/?...60-11179&dds=1
It is available from Jegs as well if you prefer doing business with them instead.