I would fill the reservoir to the proper level, start the engine then push hard on the brake pedal to see if it fades to the floor. If it fades at all I would pump the brakes and do the same things a few times. After that I would check the brake fluid level. If the level has gone down I would use a bright flashlight and look for a leak. If the fluid remains at the full mark I would try bleeding the brakes with a pressure bleeder. Low brake pedal due to pad wear is gradual rather than all at once.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas