Difference in volume of front wheel brake cylinders vs. rear wheel brake cylinders, vs. master cylinder? I'm no hydraulics engineer, but it seems to me that if you get the low pedal with the rear lines clamped and the front lines open, vs. the front lines clamped and the rear lines open (normal pedal), the problem lies mainly in the front brake cylinders, or perhaps the whole front brake assemblies. At least, that is where I would start looking. An adjustable proportioning valve might be needed, perhaps.

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