It may well be that the master cylinder bore/piston is not the right size/matched to the XTX caliper and its pistons? One of the most common misconceptions with brakes is that a larger master cylinder will create more pressure. A larger master cylinder has a larger displacement, but it takes more force to create the same pressure as a smaller bore. It will effectively take more effort at the lever to create the same system pressure. The result after adding the larger master cylinder is a lever which requires more pressure to create the same amount of braking force, hence a "spongy" feel.

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