I do not know what is the problem but if I were you I would do this test. I think the circuit is similar to my 190D. There is a connector on top of the brake fluid reservoir. Remove the connector and the warning light should NEVER come on. If it still comes on then there is something else wrong in the circuit. If it does not then may be the float switch inside the reservoir is faulty,it is giving a false reading now and then, i.e. the contact is made.

The light may also be tied to low brake pads. Check your front pads also.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.