Rear Brakes and Rear Brake Bias
In a high speed, panic stop or nearly something like that weight shifts forward onto the front axle. So the front brakes need to do more of the work than the rears because they have a lot more traction. The flip side is that with the weight off the rear tires they can't do a lot of braking without locking up.
This is all dynamic though. It won't stay that way for very long. As the rig slows the weight starts coming back onto the rear brakes and they can start to do more braking. So a properly tuned LSPV is a very good thing since it has the potential to offer maximum possible braking effectiveness in all situations. I've no idea if the range of adjustment of the LSPV on the 62 series version can actually do this.
My experiments with the one on my completely stock braked, but on 33-10.50's Mini lead to it being set to give full rear brakes all of the time. The only time that it could use less is when the shell is off and there is nothing in the bed.
Since any speed results in some weight transfer during braking there needs to be what I call a "baseline bias" meaning that the rear brakes, at best, will never equal the front brake's effectiveness. The less speed that the rig sees overall allows the front and rear caliper pistons areas to become much closer to equal. A trail only or comp rig can probably use the same size caliper at all corners.
A rig that sees highway speeds can't have this nearly 50/50 bias as that will result in the rears locking up in panic stops, or even something considerably less than that.
Classically
adjustable proportioning valves cut the output pressure to 57% of the input pressure above their knee point. Using that as a guide I suspect that the rear piston area could be as small as 57% of the front piston area. I've not done any experiments on this, so it is only theory at this time.
One way to put some baseline bias in the brake system is to use rear calipers with less piston area. Since clamping force is pressure multiplied by area, reducing the piston area reduces the clamping force, which reduces the rear braking power.
Ideally the rear calipers would have *just* exactly the right amount of piston area so that at the greatest possible loading of the rig it would see maximum rear braking power, and then an LSPV would reduce rear line pressure proportionally with lighter loadings.
Alternately an adjustable proportioning valve could be used, the operator would have to keep track of the settings and adjust it frequently, or live with reduced rear braking power and leave the adjustment at some low setting.
Another way to deal with the rear wheels wanting to lock up is with a pressure delay valve. Currently the only part that I know of to do this is the
Dan Press Ind. "Lock Resistant Brake System", a pressure delay valve that can be added to the rear brake plumbing. What this valve does is slow the pressure rise to the rear brakes. So when max pressure would result in locked rear tires this valve has slowed down the delivery of full pressure. By the time that full rear pressure can be handled without locking up the valve has allowed the pressure to rise to the maximum.