As I mentioned on the previous page, a prop valve in the front is never a good idea. As brake pressure increases, the proportion of braking done by the circuit with the valve decreases. With a valve in the front, the harder you hit the brakes, the lower the percentage of front brakes you get. This is not good, because the harder you hit the brakes, the more weight transfers to the front wheels and off of the rears. So the wheels with less traction get an increasing proportion of the braking effort as you step on the brakes harder and harder.
"Proportioning Modifications We could start this section by clearly stating that you should not modify your proportioning valve. But, what fun would that be? In all seriousness, making changes to the proportioning valve to effect brake bias should be left to those with the proper tools and measurement devices, but if you have tweaked your vehicle beyond recognition, this may be your only solution to restore a sense of proper bias to your braking system. We’ll start here with three of the most basic rules regarding proportioning valve installation and selection.
1. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed if the factory unit is still in place. Proportioning valves in series with one another can do nasty, unpredictable things!
2. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say “terminal oversteer.” Front brake line pressure should always be left alone – only the rear pressures should be considered for proportioning.
3. In all cases, the basic brake system balance needs to be close to optimized to start with. This is the only way that a proportioning valve can be effectively utilized. You should never assume that simply adding a proportioning valve will address all rear-bias conditions, as even the best proportioning valves must be well-matched to the target vehicle." https://www.apcautotech.com/getmedia/d958a29e-4ebf-41fd-931f-bf7e4451801b/brake-proportioning-valves.pdf