This light means there's an issue with your anti-lock brakes. It could come on for a variety of reasons, from a faulty speed sensor to low brake fluid to total brake failure.. Thanks for all the replies. Bled and bled again today with power bleeder and by manual method. Absolutely certain that all the air was out but then same problem again, brake pedal really spongy and rear brakes not really working (rust on rear brake discs still there after test drive).

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Detroit Axle Rear Ceramic Brake Pads w/Hardware & Cleaner
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Mercedes Automatic Window Reset Restore Functionality! YouTube
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DIY Mercedes GLS 450 Brake Fluid Flush YouTube
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Mercedes Benz Gl450 Transmission Fluid Change
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Troubleshooting Mercedes Windshield Washer Problems MB Medic
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2007 Mercedes Benz GL450 GL class X164 Humming noise rear
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The Ultimate Guide to Understanding the Mercedes Benz GL450 Fuse
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MercedesBenz How to Check Car Fluid Levels (Transmission, Brake, Power
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MercedesBenz GL450 A/C NOT BLOWING YouTube
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Mercedes Gl450 Check Coolant Level
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GL450 Brake Release Guide Repair YouTube
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2009 Mercedes GL450 X164 Brake controller install with harness YouTube
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Low Brake Fluid Warning Circuit Diagram
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MercedesBenz X164 Engine Cover Removal (20072014) GL350, GL450, GL550
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Mercedes Benz Gl450 Transmission Fluid Change
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mercedes gl450 parts diagram
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For 20072012 Mercedes GL450 Brake Rotor Rear Power Stop 87722XWXR
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Mercedes Gl450 Air Suspension
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Mercedes Benz Gl450 Transmission Fluid Change
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How to replace brakes and rotors? 2012 Mercedes GL450 YouTube
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How YOU Can Replace The Rear Brakes On Your Mercedes
Detroit Axle Rear Brake Pads w/Hardware + Brake Cleaner
Detroit Axle Rear Brake Pads w/Hardware + Brake Cleaner
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Mercedes Brake Switch Replacement Brake Warning FIX YouTube
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Mercedes GL450 Brake Light Bulb Replacement YouTube
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Mercedes Benz Gl450 Transmission Fluid Change
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Accidentally ran brake test when trying to set gl450 in
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Mercedes Benz GL450 oil change, filters replacement. YouTube
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Mercedes GL450 Trans fluid let it drip to get more
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2007 Mercedes Benz GL450 GL class X164 Humming noise rear
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Signs and Causes of Low Brake Fluid Sun Auto Service

Well more by luck than judgement. I got rid if my ABS and ESP Currently unavailable messages. And the Parking Brake See Owners Manual message. - by Resetting the Electronic Parking Brake. All seemed fine.other than my defective low voltage battery issues. (But now convinced thats not.. It's part of a 1-million-vehicle recall worldwide over a defective brake booster housing unit that could make GL-, R-, and ML-class models unable to stop.