Brake sensor is on, yellow.

Hello, are you saying there is a brake system warning light illuminated yellow? If so, this warrants professional diagnostic testing, as there is data available to assist in knowing what is wrong with your BMW. Secondly - if your vehicle appears to be slowing when not accelerating, it could be an indication of an electronic or hydraulic brake system problem. Testing is needed to see what is happening. If you leave this go, you could cause the brakes to overheat and have a bunch of additional problems.