The purpose of the Ford brake master cylinder is to create and supply hydraulic fluid pressure to the brake lines and hoses in the system. This makes it possible to move the pistons in the brake calipers and wheel cylinders so force can be applied to the drum and rotors surface which brings your vehicle to a stop. Under the top of the brake master cylinder are two reservoirs. The front one is used for the rear brakes and the rear reservoir is for the front brakes. The Ford owner will also notice that the rear reservoir is larger than the front one. The reason why is because of the brake biased which provides twice the amount of braking power to the front brakes as it does to the rear on your Ford vehicle. When a Ford brake master cylinder fails, it is because of the seals on the piston. Unlike when other seals in the braking system fail and there is a visible exterior leak, the seals in the brake master cylinder can not been seen on the exterior of the device. These seals are on the piston in the bottom of the brake master cylinder preventing the pressure from being forced into the brake lines and hoses. Any Ford driver will notice this is occurring when the brake pedal is being applied and little to no force is felt. The brake pedal can also go all the way to the floor but no braking power felt. When the seals of the piston leak or totally fail the replacement of the brake master cylinder is required or the vehicle will not be safe to drive on the road.