5-Series (E12,E28, E34, E39, E60)Chat relating to the BMW 5-Series of all generations. Specific models include: BMW 518, BMW 520, BMW 520i, BMW 530i, BMW 528i, BMW 530i, BMW 518i, BMW 524d, BMW 525i, BMW 525e, BMW 528e, BMW 540i, BMW 535i, BMW 520d, BMW 525td, BMW 525d, BMW 530d, BMW 525i/xi, BMW 530i/xi. (BMW 5-Series Forum)
Like what Torque said, it's probably your brake pads wearing out or the wear sensors themselves are shot as well.
I also highly recommend replacing your brake lines with a braided stainless steel line as well. They do not flex as much internally under braking pressure and also last a lot longer than the OEM rubber lines. They also cost about the same as well which makes it a cost effective upgrade.
You will also notice your brake pedal feel is much firmer
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