I have been faffing about talking about swapping the master cylinder on my old XR6 for about 300yrs. I have finally done it. The lever was getting so spongy. The OE master cylinder from the XR was too small for the 4 piston Brembo fitted to the bike.
A master cylinder from a CBR6 has done the job. I need to fit better quality pads, the new one I have fitted are for a scooter!
If anyone else is in the same boat, just do it!