Strange juddering at low speeds on rear brake -Diversion 900

[Cheeseybeaner]

Wondering if anyone has any ideas on this...I recently had the rear master cylinder rebuilt with new seals, fitted a goodridge hose and changed fluid, rear disc was also replaced around the same time.
The bike basically stops fine with the back brake, it appears to work okay but at low speeds say beneath 20mph I get a strange jerkiness as if its applying pressure intermittently so you can't really control the bike with the back brake as much as you'd like to.
There's no unusual feeling coming back through the lever when doing this and it otherwise works very well at all other speeds. I thought the master cylinder seals might have been going but have also just replaced the unit complete with a Fazer 1000 master cylinder this week.
The disc looks pretty much fine to me, if you spin the back wheel it doesn't seem to bind against the pads though it looks like a tiny flutter may be there - I wouldn't have thought it enough to cause problems.
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My bikes: 2003 KAWASAKI ZRX1200R, 1996 YAMAHA DIVERSION XJ900S

[Robby]

How well did you clean up the wheel before fitting the new disc? Alloy corrosion on the wheel can push the disc out of true. To fix, remove the disc and sand down where it seats until you have bright, clean alloy.

[Cheeseybeaner]

[Nemo]

These are shaft driven right?

Did it start doing it pretty much as soon as it was done, or a little while after?

If there's no odd feeling through the brake lever then it's not going to be the brake system itself I wouldn't have thought.

Has the rear wheel been located properly?
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Previously owned: Kawasaki KH125, Suzuki GP125, Suzuki GX 125, Honda CB125 TDC '83, Honda ntv 600, Kawasaki ER5, Kawasaki ZZR600, Aprilia RSV 1000 Mille
Now own: Ducati 848 evo

[Cheeseybeaner]

[Nemo]

Checked the caliper mounting? I forget what its call, but I think its the torsion bar or something?... Does it have one? If so is it secure? Not sure that would give a juddering feeling though..

Im thinking out loud, sorry!

What have you checked?

When you're stationary and have the brake on, and you rock the bike, is there any movement?
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Previously owned: Kawasaki KH125, Suzuki GP125, Suzuki GX 125, Honda CB125 TDC '83, Honda ntv 600, Kawasaki ER5, Kawasaki ZZR600, Aprilia RSV 1000 Mille
Now own: Ducati 848 evo

[Robby]

If the mating surface on the wheel has corroded, it will push the disc out of true and make it feel like a warped disc. This can happen with the disc fitted, corrosion happens slowly and has enough force to push a disc out of true. It can cause juddering/pulsing.

[Cheeseybeaner]

[Ariel Badger]

If you have a Dial Test Indicator (Clock) mount it on the swing arm and test the disk for true.
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Bikers make great organ donors, get 115 on your licence today.

[Fizzer Thou]

You do not say that you replaced the pads at the same time as the disc.If the old disc was worn and grooved then so will the pads be.If they are contaminated or worn unevenly then they should be replaced.

Was the new disc thoroughly cleaned with brake cleaner before installation?
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Cheeseybeaner]

[Cheeseybeaner]

Disc from ABE now fitted on the bike and the juddering's gone completely...
Would love to know what I actually did wrong with the EBC disc, is it possible that the corrosion could have contributed to warping/uneven pad deposits on the disc enough to leave me with a disc with bad run-out?
Trying to be as careful as possible bedding in this one, 200 miles at light to moderate braking then 10 heavy braking runs from 60 to 20 sound about right?
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My bikes: 2003 KAWASAKI ZRX1200R, 1996 YAMAHA DIVERSION XJ900S

[Polarbear]