Whats the best master cylinder to fit 315, iv done every other check but brake is still crap, so time to ditch the grimeca master, which masters will fit my bike??

Not sure what the Grimeca one is like on the 315 but I replaced the one on my old Rev3 with a Brembo one from some Aprilia road bike. It was also cheaper at £35. I found my local Brembo stockist and took my old Grimeca along and within half an hour we had found an almost perfect Brembo match

was that a brand new brembo master or a second hand one?

The only thing that made my old single pot brake caliper behave was a new AJP master cylinder.