Fluctuating front brake!

[james900]

Hi Everyone,
I made my first Ducati purchase a couple of weeks ago, a mint condition 2000 reg M900. I've done about 200 miles and love it to bits. One small problem is it had an advisory on its MOT (but passed) for a fluctuating front brake. The front brakes are super sharp at slow speeds and its quite noticeable that it fluctuates slightly and makes braking a bit juddery.
Has anyone had experience of this before? Not sure if warped discs are one of the only answers?
Any help gratefully received.

James

[gregski]

I have the same problem on my 620

first chesk the pads and calipers if one of the cylinders stuck then you get fluctuating brake if they are ok then it is possibe that one of your disck is bend or like in my case one of my disck have a different surface

[Bunny]

Sounds like a warped disc. If you can get the front wheel up in the air, you can spin the wheel slowly, and watch the disc as it enters the calliper. If it's warped you will see it touch against the pads.
Can't be too bad if they didn't fail it

[Starter Sprag]

It may be your disc 'bobbins' are full of brake dust and crud

Put a nut and bolt through bobbin, then grip using battery powered drill

Set to SLOW speed, spray liberal amount of brake / contact cleaner to flush all crap out

Do this to each bobbin in turn, both sides

[utopia]

Had exactly the same on my 2000 M750. MOT advisory too....twice. Still haven't got to the bottom of it. Checked both discs for runout (still on the bike) with dti ..both inside and out...all well within tolerance. Cleaned bobbins as Sprag suggests (tho just used a long bolt with two locknuts and turned the bolt by hand/spanner)...still no better. I lightly scoured the disc surfaces with a scotchbrite washing-up pad, and again with fine grade wet/dry paper.....still no better. I've removed and cleaned the calipers recently too, tho not specifically trying to cure this fault. And I've changed the brake fluid recently. I'm interested in Gregski's comment about "different surfaces" but can't imagine how such a condition would arise, unless maybe through uneven work-hardening or tempering of the steel, but why "uneven"....? I did once hit a pothole with enough force to put a small ding in the wheel rim, which I had professionally straightened....wondering now whether the spindle's bent, but the discs do still run true, so I'm not convinced. The problem is not really noticeable under heavier braking on mine....it just grabs a bit under light braking, typically when coming to a halt at traffic lights. My mot tester is a sound guy, and he blamed the discs...said he'd cured the same fault on another bike (not a ducati, but maybe with similar brembos, I think) His actual words were "they're **** discs" but....BREMBOS..?? I dunno what grade of steel they are, but I'm assuming they're some form of stainless, and they're magnetic which I think means they fall into a broad group called "Martensitic" tho my matallurgy is vague and ends there. Stainless steels are notoriously prone to problems when their surfaces are worked. The threads on SS bolts and nuts are generally machined a little looser to cope with their tendency to seize, which is why they're not always a good idea in soft aluminium alloy engine cases etc. So maybe there's some support for the "different surface" notion. In the meantime they are still the best brakes I've ever owned. I remember 1970s discs which had a 1or2 second lag in the wet. Now THATS a **** disc. ....but they passed the mot.

[boris]

I had an advisory for the same thing 4 MOT's ago. Nothing was changed at the front end and it wasn't picked up again on my last 3 MOT's, latest one being 2 months ago?

[Nickj]

Take them off and go over the surface again, get a nice scoury finish all over. The kind that would have you killed if you did it to the best stainless pans ;) and looks like light machining marks.
Also go to lidls and invest in 2 cans of brake cleaner. Use one on each and shift all the dust and rubbish. Theyre all of a quid or so each so you can be generous!!
Now reassemble and find somewhere quiet.
Get up to between 60 - 80 and brake down to about 5, it's important that you use pretty positive braking (I mean hard but not to the point of locking the wheel) don't stop just get back up to the same speed and do it again.
You want to do this 6 - 8 times.
By this point the disks will be hot, Like VERY hot!!
Sit down for 15 mins and let them cool off.
This ought to get a layer of pad material deposited on the disk and get a more uniform surface layer. If the leading edges of the pads have a little grey band down by the disk surface then they've got hot enough.
Hopefully you've just re-broken in the disk rotors.
If it doesn't work then you've probably got a small area where the cast irons structure has changed a bit when the disks were new and they got hot first time round but not hot enough to make a uniform change all over the disk. If the above doesn't work and you want a cheap fix then you'll need a bit under a thou skimmed off the surfaces before you do the brake brake brake series again.
If you've been using relatively soft pads designed for cast iron then they have a habit of transferring a bit of material randomly to the disk surface when hot which gives a tiny thickness variation (micrometer thick) and feels like a warped disk. Again the above brake brake brake sequence should get a more even layer and kill any relatively thick areas.

[Nickj]

I had an advisory for the same thing 4 MOT's ago. Nothing was changed at the front end and it wasn't picked up again on my last 3 MOT's, latest one being 2 months ago?

That was probably a layer of pad material on the disk surface, a good hard run on the bike tends to shift it.
Until the rotor and pads get really hot it'll just stay there, sometimes you have to get the pads up to the temperatures where they are just starting to fade to shift it .... Unfortuntely that tends to be when it builds up so it's a bit of a balancing act.

[Nickj]

Oh and the rotors, you notice that if you put it away with damp pads by morning there's a rust patch, sometimes the pads even feel sticky.
That'll be iron not stainless steel

[james900]

This is one hell of a forum, thanks for the advise guys. I'll give it a go this weekend and see what happens.

[utopia]

Thanks Nickj. Sound advice I think. Must admit I hardly ever use the brakes to anything like their full capability on the road. Its usually just a one finger jobby, given the engine braking an' all. And I've never done a trackday, so my discs have probably never had a proper workout in their 7500 mile lives. I'll definitely give it a go. Ta for the Lidl tip too. Not sure that its neccessary to remove the discs to clean them though, as long as you avoid dragging any loosened gunk on to the pads. Last time I just turned the wheel in stages and cleaned the bottom bit, with a towel over the rim and tyre. Saves any chance of misassembly afterwards, or overstressing the bolts, and lots of time. Its worth marking the discs if they are removed though, so as to get them back in exactly the same place ie left/right AND radial position. As for the rotor material, I thought you had me there, but I checked my genuine Ducati workshop manual and it says "drilled steel", but no more than that. Suppose it could be a translation inaccuracy but then again, stainless steels do still rust to varying small degrees, depending on the grade. And I believe that the magnetic ones (turns out thats Martensitic or Ferritic) rust more readily than the non magnetic ones (Austenitic). That may be academic though, cos I think you're maybe on the right track with your advice, whatever they're made of.