After hearing much discussion and some misinformation on this subject I thought to dispel misunderstandings and rumors by the application of factual data. So I removed the valve from a spare car and disassembled it.
The valve comprises an:
The cast iron body was free of corrosion but full of gunk as disassembled. The casting was very smooth and the bores were polished to a high degree indicating a high nickel content. There were two coaxial bores of 1.00 and 1.25 inches with the space between them vented into the mounting bolt hole. The piston was of machined aluminum with a mount in one end for the seal and grooves for the "O" rings and a hole through it that held the pin. The "O" rings are 1.00x1.25 and .75x1.00 respectively and should be commonly available. The seal has the following inscription molded in a circle "FAG 1-64 GAB GIRLING 2232660139 ".
As mounted the ball must roll up a 26 degree slope relative to initiate functioning. The piston areas have the ratio of 16:25 or 64% When pressure is applied fluid flows through the piston actuating the rear brakes at the same pressure as the fronts. When the car reaches the deceleration rate of .5 Gee or is going down a 26 degree slope or any combination of the two factors then the ball rolls up the ramp sealing against the piston seal. Any further increase in input pressure is attenuated by the ratio of the piston areas or .64 (64%).
As an example let us say that a car is braked in such a way that at 1000 Psi line pressure the car decelerates at .5Gee. An increase in pressure from to this point to 2000 Psi will result in 2000 Psi to the front and (1000 + 640)Psi to the rear. If the line pressure went to 3000 Psi then the fronts see 3000 Psi and the rears 2280 Psi.
The reduction is fixed by the ratio of piston areas and the knee point is set by the slope of the valve to the car. Note that this is directly related to weight transferred to the front wheels rather than just the line pressures. As a result under low traction braking (Ice) the braking effort is equally divided between the front and rear while under high traction braking (Dry) at high deceleration rates with great weight transfer braking effort is shifted to the front.
There are NO residual pressure valves or anything else in the line.
The function of the pin is to lift the ball off of the seat in the case of the piston reaching the limit of travel. Then the fluid simply flows through the piston as if the valve was not there. The only adjustment to the system is to change the angle of mounting relative to the car. A steeper angle raises the deceleration rate at which the valve comes into play. As the valve is mounted by a single bolt and the angle is set by a pin extending into the fender well it would be relatively simple to add another hole to mount the valve at a steeper or shallower angle. A device providing a cockpit adjustment is not unreasonable either. I believe that the reason that this device is removed entirely or replaced with a pressure valve is a lack of understanding how this valve works and what it's advantages are. Hopefully this article will dispel some of the mystery surrounding it!