The portortioning valve has a sliding cylinder with seals on both ends.
The opposite ends are exposed to the front and rear brake lines.
There is a notch at the top middle of this cylinder and a brake warning light switch is on the top of the valve with it's probe in this notch. Both the emergency brake switch and this P valve switch is connected to the brake lamp in the speedo in a CJ. The line through the lamp is hot and the switches ground the circuit.
If either of these are grounded the lamp will glow.
If you were to have a leak or serious pressure difference between the front and the rear hydraulic brake lines the cylinder in the P valve would move over and the switches probe would be pushed up, grounding the circuit and activating the brake lamp as a warning of brake system failure.
The reason the manual sez to hold the cylinder's notch with a tool is so the cylinder won't slide during bleeding.
Van, the brake man sez "don't worry about it" because once the pressure is equal the cylinder usually pops to center anyway.
It is advisable to bleed the fronts occasionally while doing extensive rear (the first in order) bleeding as this will help keep equal pressure in the fore and aft lines which will then in turn help prevent the P valves cylinder from sliding off center.
And that's about all I know about that ...[img]/wwwthreads_images/icons/smile.gif[/img]