Hi All,
I've just fitted a pump/master cylinder from a gsxr1000 K5 to a 1000 K4...
The braking is awesome and very progressive, but sometimes when I brake on an uneven surface I get some really strong vibrations coming up the forks, not enough to make me come off the bike but enough to force me to release the brake lever for an instant to make things calm down and then I can apply the brakes again, this never happened on the original setup.
I can't tell exactly what the problem is, I'm thinking suspension as it does only happen on some uneven surfaces, I don't think it's a brake pad problem....
any help is much appreciated, I don't want to revert back to the original master cylinder as this new setup really has made a huge difference....

i had this problem before. the problem i had was a VERY small flat spot on my tire which had caused a uneven wear pattern all the way around my tire. when i replaced the tire, the occasional vibration went away. :cheers maybe you've got something similar going on. :confused

actually, that's not a very good swap.

The stock '03/'04 mc has a very small piston diam... 14mm. The new radials have 19mm pistons. Basically, when you pull the lever now, you're moving ALOT more fluid for each unit of lever movement. What this will give you is a much harder, stiffer lever, but much less progression. It also gives you less mechanical advantage than if you had gone with a smaller piston diameter, though your new combination does net out to provide slightly stronger braking. However, I think that the lack of feel and progression at the lever would outweigh this small benefit.

The mc's work well on the new bikes they were designed for because the caliper piston area is larger. You shouldn't change out brake system components without considering the whole of the system.

If you want a radial that would work well with your present calipers, I'd go with one off a CBR1000 (and possibly 600RR). the cbr mc has a 17.5mm piston that would give you much improved braking power and more progression (more lever travel).

Oh - and I'm not saying that your new MC is necessarily the problem with your brake feedback... only that it's not a combination I'd suggest running. You could have other mechanical problems, or maybe you just haven't braked with the same force over this particular stretch of road. But it is a good possibility.

thanks for the help guys, I'd read that the calipers on the K4 were the same as the K5... still I did pick up the MC from ebay for next to nothing, so no loss... will start looking for a MC from a CBR as you've suggested....
any excuse to take things apart especially in the winter months is always good:)

cool deal, bro. Once you get one hooked up - write me and let me know how you liked it! :cheers

next week I'm having new springs fitted in the forks, will get my mechanic to take a look...
I started getting vibrations immediately after I changed the MC so I'm thinking you're right... will let you know how things turn out...