I understand that, but the ports are filled with white powdery substance so I am sure the veins are clogged and the valve will not come out I know it should slide to trigger the light but I have tried everything to remove the plunger but it is stuck forward. If you have any suggestions on how to remove the plunger I will try but if the ports are full of this substance I am not sure I can clean all the vanes thoroughly. And I think looking at your pictures if that valve slides forward it will restrict the flow to the rear brakes or backwards it will restrict the flow to the front brakes and that is how the light is triggered. I have experienced this also happening in older cars wheel cylinders when they sit for long periods of time since dot 3 fluids and moister do not work well together I have had to beat the pistons out of the cylinder.
I think this is a good explanation on brake valves, very interesting