The master cylinder can leak, however, brake fluid leaks behind the front wheel and in front of the rear wheel would not be associated with the master cylinder. I would check the lines into the brake calipers or see if the fluid is leaking out of the calipers which would indicate a brake piston seal is bad. If the X is running and you continue to push hard on the brake pedal will it go to the floor?

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