I run one of these valves, on My M-679 and on my FC Tour Jeep, although I run large 4 wheel Disc Brakes, the principle is the same on drum brakes. From in my seat, in the cab, I can fine tune with a small twist to turn the knob, I get an exact control and adjustment of the amount of pressure to the rear brakes, to prevent worries during panic stops at high freeway speeds in the rain.
Although I run dual master cylinders, the adjustment, allows me to increase or decrease rear pressure, depending on loads, speed, rain or wind. It will work the same on a stock FC master cylinder. leave the front brake line as it is, it will have full pressure to the front brake drums, use this proportioning valve only between the master cylinder and rear brake line. On your system, the adjustment restricts at your requirement, only to the rear pressure while the leftover pressure will be there to go to the front drums as it equalizes the bias pressure.
On my M-679 Van, around here in AZ I thought my brakes were just fine, I thought everything was just fine.
On the way to Morango, IL, I needed to use the knob a few times, to get better control, if it is really windy, at 75 mph and the rear brakes are grabbing, the front will feel fine, but the back feels loose and sloppy from wind shifting from different side to side, from canyons or mountain wind loads, against the side of the van. On the way back, we had the worst rain storms. luckily I had it all figured out, it is dangerous without it, at any speed. New Mexico, I-8, is 80 mph, but the trucks are running faster at over 90 mph, like running with the bulls, in the rain.
I don't usually drive the M-679 against the wind and rain, in high speed highway traffic. But it is a nice to have a brake part that works this well for me.
: Even with the addition of 300lbs of sand in
: plastic buckets in the rear bed stopping
: quickly still locks up the rear brakes. I
: have considered reducing the tire pressure
: in the rear tires and increasing the
: pressure in the front by ten pounds. Any