I'm starting to really think about swaping out the Rear Caliper from a 250r onto my 400ex. I want to go to the twin-piston setup for stronger braking and improved pad life due to almost twice the braking surface area. I'm pretty confident I can fabricate the mount (if needed), but my main concern is the rear master cylinder. Will it have enough power to push the two pistons? Is the 250r unit a stronger pump? If it is i'll switch over to that too.

 

you won't need the rear master cylinder it will be able to handle both piston's easy it's just the caplier they changed