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If brake pedal goes to the floor and the master cylinder is full of brake fluid with no leaks in the system, the master cylinder is bad. It has what is referred to as an internal by-pass. One of the internal seals has failed and as a result when you push the brake pedal down instead of sending fluid and pressure down thru the brake lines, the pressure bypasses the internal seal and the pedal drops to the floor.
If you want to do a test to confirm, you will need to do a brake line test. With master cylinder resevoir full and system bled, clamp brakes hoses at each wheel with brake line clamps(available at auto parts stores) or use vise grips at each hose(do not over clamp with these or you damage hose). With all 4 brake hoses clamped off, test brake pedal, it should be firm. IF pedal drops, the master cylinder is bad. If pedal is firm, you will need to remove 1 clamp at a time, and test pedal. When you come to the wheel that when clamp is removed and pedal drops to the floor, you have found where problem is.
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