Odd brake problem on CB500s. Unexplained pulsing at lever.

[stinkwheel]

Kats CB500 failed its MOT due to "Excess fluctuation of braking effort on front brake.".

Sure enough, when I tried it, there was an obvious and pretty severe pulsing on the lever, especially noticeable at low speed. It wasn't doing this a month ago, it hasn't been crashed and she doesn't even have a disc lock.

I have had a new tyre fitted since then and fitted a new disc on the assumption it was warped.

Rode it 10 miles to the MOT and it was, if anything, worse. The MOT man had the new pads in for me. I fitted them and it improved a lot, enough to pass the MOT but still not perfect.

We whipped the wheel out and stuck it on his balancing jig. The bearings are smooth with no play, the wheel is perfectly balanced. We put a dial guage on the disc, it's straight. There is a VERY slight buckle on the wheel but you need the dial guage to see it, well within service limits.

The brake pistons are clean and free moving, you can push them back with hand pressure. As are the slider pins, free moving, no noticeable sideways play. I've bled through clean brake fluid and there is good pressure at the lever.

So. Wheel is straight. No runoff on the disc. New pads, new tyre. Calliper in good order. Fresh brake fluid and a general clean up. Still getting pulsing at the lever.

It has both the mechanic and I scratching our heads. He says he's seen this before on two other Hondas (an early 'blade and a deuville) but never heard back what, if anything, sorted it in the end.

I wondered if the "posts" on the wheel casting that the disc bolts onto are somehow twisted or uneven but surely this would show up as runout on the disc?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Itchy]

My CBR600 has an identical problem, the discs are straight, and I installed the brake pads and pistons/seals myself all new ones.

And yet it does it, feels like warped discs but they aren't.

Speedy's mechanic was also scratching his head at what may have caused this. The CBR of course did sit under a tarp inside a box in storage for a year though while I was away.

I overhauled the callipers again cleaning them out carefully and it still did it, I put the old pistons in and it still did it, so quite simply I ignore it.
____________________
Spain 2008France 2007Big one 2009 We all die. The goal isn't to live forever, the goal is to create something that will. In the end, your life will flash before your eyes. Make sure it is worth watching.

[TUG]

It keeps popping up about corrosion behind the disc on the rim or such, are the rivets free? you can clean them with a drill, bolt and nut with some WD-40 or GT-85.
____________________
Haz ER-5, innit!

[stinkwheel]

The mounting points are clean as a whistle. It's a brand new, one-piece disc which I torqued down very carefully, crosswise, in stages (10, 20, 30 then 43Nm)
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Rowey]

Could the master cylinder seals be at fault?

Or the brake lines old and bulging?
____________________
ZX6R C6F.
Never argue with an idiot. They'll only bring you down to their level, and beat you with experience.

[The Artist]

Calliper bolts clean and free of corrosion?

[steven_191]

you say you checked run out on the wheel with a dial guage?

maybe try checking the disc mounting points with the guage too. if they are out of line then bolting the disc to is would cause the disc the warp too.

or try rotation the disc around a bit and bolting it back up.

[iooi]

Seals going in master cylinder ?
____________________
Just because my bike was A DIVVY, does not mean i am......

[stinkwheel]

Someone suggested it might be a warped calliper making the pads strike the disc at an angle. So the leading edge of the pad hits first, braking somewhat. The whole pad then flattens itself to the disc, braking more. Setting up an oscillation.

Not sure how to check though.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Grimnir]

You say you have used a dial guage when the wheel was on a balancing rig - have you tried doing that with the wheel in the forks? As you have checked all the other things I would have suspected could it be not running true when actually on the bike? Not sure exactly what would cause that though...

If / when it does get sorted, make sure you post the results on here - it is a weird one

[Rowey]

[stinkwheel]

[el_oso]

if you still have to old pads check that they have worn evenly
have you check that the forks are straight. i.e the bolts holding the stanchions in are level with the top of the crown or the correct height above the crown?
____________________
Duke 390
Previous: '05 XR125L | '96 XJ600S Diversion |'05 Suzuki GSXR1000 | '05 Honda CBR125-R | '97 YZF 600R Thundercat | '11 Honda CBR250
Car: Jeep Wrangler 4.0L