Im not sure what that sensor does but when I was bleeding my brakes a couple of years back my brake warning light in the cluster came on and wouldnt go out. I thought maybe I had screwed up one of the pad sensor wires but they were OK when I checked them. So went looking at the other sensors. When I got to this one I noticed the little nipple in the center of it right by the plug( hard to see in the pic). In that nipple there was a tiny little button that was sticking up. I pushed it back in and the light went out.
So my thought was that is a low pressure sesnor that tripped when it saw the low pressure while manually bleeding the brakes. I pushed the button in and it reset.
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