The brake light serves 2 purposes. One is to tell you the brake is set and the other is that the brake fluid is low. Your brake reservoir is clear (opaque) and you should be able to see the fluid level. If your fluid is real dark the it needs to be changed, but that's another subject. If your fluid is low, lust add some. Be VERY careful not to spill any. Brake fluid is very corrosive, especially on paint. Use DOT 4 brake fluid only. DOT 4 can handle higher temps than DOT 3. DOT 4 can be bought at any Walmart. I have actually gone to 2 different auto parts stores and they didn't have DOT 4.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member