Proportioning Valve Question

[stope4]

First off, I'm not a brake expert, so my terminolgy and how the valve actually works may be a little off, but I'm preety sure this is what the button on the front of the proportioning valves is for.

There's a piston on the inside of the proportioning valve that can move towards the front or rear of the proportioning valve. There is a machined grove in the middle of this piston. If you remove the brake switch on top of the proportion valve, you will see that it has a spring loaded protrusion that is supposed to reside in the grove on the piston. Sometimes the piston shifts towards the front or rear and is no longer centered by the brake switch. I guess this shift occurs when the fluid pressures for the front and rear of the brake system are unequal, like when you are bleeding brakes. To recenter the piston, you can press the button under the rubber cap. Or, to prevent the piston fron shifting while bleeding brakes, you can keep the button depressed during the bleed process. Warning, pressing the button under the rubber cap requires a c-clamp or something similar. Your fingers or thumb won't be able to depress it under the extreme pressures of the hydraulics in the brake lines. I'm pretty sure that when the piston shifts, the brake light comes on.