The condition is called bypass leaking. When you step on the break, the fluid is not forced into the wheel cylenders. the majority of the pressure that does not by pass the leak back into the MC goes to the nearest wheel cylenders but not in the same amounts becasue of the wear and tear on the various sides. That causes the pull to one side or the other. The By pass leaking can occure in either the front ofr real but usually isn't noticed in the rear.