I have a choice to make now. I currentally have a stock 1999 Yamaha r1 master on there (5/8), and today on the way home, i picked up a barely used, like 10 miles radial master cylinder off a 2007 gsxr750 (3/4) for .50 cents (god i love yard sales). My question is, which would you use and why? And yes, the radial master works and builds pressure.

 

The f4i has a 5/8 MC stock. Did you see any improvement going with the R1 master?
Upgrading to a 3/4 would give you a nice improvment.

Though i have no idea how the gsxr MC would fitup to the f4i bars.

 

idk but ill know soon it looks like. and i did have an improvement with the r1 master. My stock f4i master wasn't working right, had one spot where it would get stuck slightly until i put a little more pressure pulling to move it past that spot, and at the point it would go to far and lock up the front end. Needless to say, VERY unsafe... plus the r1 has awesome stoping power. So compared to what i had, was a HUGE upgrade.