On the cars I own, the brake light comes on and stays on when the fluid is low. I don't see the value of a bulb that doesn't light when fluid is low! I could be wrong, but it sounds fuse or electrical.

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John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E