Hello Leon,

I have had the cylindrical sensor in the washer tank go belly up on me in the past. I believe it works via a magnetic switch (floating donut with a magnet in it that slides up and down along the switch - thus relaying an empty or full WS fluid level). It is possible that 1) the switch is gone and by coincidence the light comes on as you brake or 2) the donut is jammed close to the bottom of the sensor assembly (due to dirt or crud build up) causing small motions such as braking to intermittently trip the sensor. It might be useful to take out the sensor and examine it. If the center switch portion has a dark fluid in it (like mine) the sensor has probably bit the dust. At this time, you can also examine the motion of the donut assembly by placing it in a tub of water.
If it is jammed, take a thin screw driver and pry off the white bottom cover to clean out the assembly. This cover then can be pressed back on for further use.

Further to these two suggestions, if you have an ohm meter you can look for a change in resistance depending on the position of the donut to confirm if you need to replace the +$40(can.) sensor.

Tim