Could you elaborate on what the indy mech found/recommended? How was the fluid contaminated? If it was water seeping into the brake lines, I would flush it a few times to get all the moisture out. Also, are the rear brakes not working because the fluid is contaminated or because the brake calipers are frozen? If the calipers are frozen, you can rebuild them or buy remanufactured ones to replace...then flush and bleed. What exactly did your indy mean by "replacing the whole system"? Brake lines, rotors, pads, fluid, calipers?

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2000 Z3 2.3 Steel Gray
2004 X5 4.4i Jet Black