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1. Clamp off the brake hoses to the front and rear wheels and retest to see if the pedal still fades. If it does not, remove the clamps one at a time and see which is allowing the pedal to fade and service as necessary.2. If the pedal still fades, remove the brake lines from the replacement master cylinder and put plugs in place of the brake lines and retest for pedal fade. If the pedal still fades, replace the defective master cylinder.3. If the pedal does not fade, about all that is left short of a leaking brake line, is a bad Hydraulic Control Unit (HCU).4. Remove the dust cap of the accumulator at the HCU- these will be two rubber caps or plugs located below where the brake lines connect to the HCU. Take a straightened paper clip and put it in the small hole that is under each dust cap and have a helper apply the brake. If either paper clip moves out as the brakes are applied, the HCU is internally leaking and will need replacement.
Also, you can remove and block off the hydraulic control unit. If the pedal no longer goes to the floor, perform electronic ABS bleed procedure, if this does not resolve it, suspect hydraulic control unit faults.
Have you had the anti lock brakes bled with a compatible scan tool?
I wanted to see if you need any additional information.
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