Sureal goings on at the rear!

[Mr Gazza]

In 42 years of motorcycling I have never had an MOT failure. At one time I even used to take my AJS & Matchless club mate's bikes to the testing station after giving them a personal once over (Just the bikes..).

Last Saturday was MOT time for the Monster and I presented it sparkling as usual and was suitably flattered by the testers gasps...He owns a 748.

Then he started looking worried as he turned the rear wheel, and bashfully said to me " it won't turn backwards"..."Ah that's Ducatis hill start device" I told him.
But seriously the wheel would not turn backwards...WTF? This was so perplexing that soon there were three of us turning the wheel and jiggling the caliper.
It seemed that the caliper..The fully floating set up,,,was racking over a little and jamming the disc somehow. If we pulled it outwards at the rear end with the hose it would clear and revolve freely.
We couldn't fathom what was going on, but needless to say he gave me a failure as something was clearly not right.

Back home I stripped and cleaned the pivot bushes for the floating caliper bracket and cleaned the pads and caliper....Everything looked perfectly normal and complete.

There is some play in the bush assembly allowing the caliper bracket to twist visibly when asembled and tight on the axle. Also the outer pad has worn slightly wedge shaped and the inner has too, in the opposite sense but much less so.
The thin end of the outer pad is the forward edge, facing the normal direction of rotation.
When the wheel was pulled round backwards the caliper tipped in at the rear and the pad must have been acting something like a pawl in a sprag clutch?

Anyway I'm changing all three pivot bushes in the assembly just to be sure and also the pads.

Now the brake works very well indeed in the normal sense and doesn't drag. The pistons expand and retract when the lever is operated..All good.
Except that I found it next to impossible to push the pistons back to make it easier to get the pads over the disc, even with the resevoir cap off. In the end I let some oil out of the bleed nipple whilst pushing hard on the pistons and this improved things dramatically.
Strangely the pistons keep returning to the more retracted position after every lever application, where as I thought it would pump back to where it was?
I'm now wondering if the master cyl filling port is blocked or masked by the piston?
I don't really want to fork out for a new Master aswell, in fact the greater retraction is working in my favour at the moment...Any Ideas?

I'll just be talking a load of crap I imagine, but piston returning to original position could point to the piston seals could it not? A kinked/nipped seal would explain the piston being hard to push back. Did you have them apart when they were off?

[utopia]

Its a weird one alright.
I haven't studied the master cylinder design closely or stripped one down to have a look but the over-retraction of the pistons does sound like the reservoir is isolated somehow and the pistons are therefore being sucked back.
I'm not familiar with the fine detail of the floating rear brake set-up either, but I would be inclined to check that everything lines up laterally like it should (once the play in the bushes has been eliminated).
When I was fitting the ally swingarm I assembled everything but left the pads out of the caliper. Its easy then to see if the disc is sitting central in the caliper body.
I guess I'd be checking wheel alignment too, but I can't really see that being far enough out to cause sufficient misalignment at the disc ... not that I would expect such error on your bike anyway, but when you're clutching at straws they all look worth grabbing.
Not a huge amount of help there I suppose ... but I'll be pondering.

I can't help thinking that its less likely that both the "won't turn backwards" and "sucked pistons" are two independent faults occurring coincidentally .. more likely that the two faults are somehow related.
BUT I can't see the link at all .. yet.

[Mr Gazza]

Thanks for taking the time to give it some thought chaps.

Ross, I have had each piston all the way up and down their travel, but with my apparently, closed circuit system, one comes out as the other goes in....No I haven't had it apart, I would never do so unless I had planned a complete re-build with new seals at the ready.

Utopia, I will try and look at alignment when the new bushes and pads arrive, but rather difficult to actually check in reality. There is nothing to adjust in any way. It goes together as is and that's that.
Wheel alignment cannot effect calliper/disc alignment either, as they are common to the wheel spindle, in the case of both floating and non floating set ups.
It is possible that my calliper bracket is twisted? this was brand new from Ducati only about 4/5000 miles ago ( the powder coaters lost my original). The middle bush, pads and torque rod ends were all changed at the same time....And yes I cleaned all the powder coat from all critical areas.

I think the master may have been sealed or restricted from the res for some time. Maybe I only just managed to get the pistons back enough when fitting the new pads? The pads have sometimes shown signs of dragging a bit, sometimes clicking in the calliper when turning the wheel so perhaps they have been tight enough to wear in the wedge shape and also load the bush enough to cause problems there too.
The irony is that it seems to be working really well assembled back dry, with the pads nice and clear of the disc. But I don't have the balls to take it back to the tester without really addressing the problem...Knowing my luck the pistons would move out properly on the journey there and bring back the hill start wheel syndrome again.
Looks like the master will have to come apart...I hate messing with hydraulics!!

[Mr Gazza]

Shortly after my previous post, and reflecting on the matter. I ordered a new master from Moto Rapido.
I couldn't bear the thought of dismantling my master to find it un-repairable. So the new one will remain in it's wrapper until mine is condemned or otherwise...More news on that story later...

[utopia]

Yep, agreed about the wheel alignment.

In the unlikely event that you do detect any misalignment, I could always make you the odd shim/bush/etc to correct it ... probably.

Another semi random thought ...
My original rear pads had stainless backing plates which cover about 60 to 70% of the piston contact area .. on the "downstream" side if I remember correctly.
This presumably causes the trailing edge of the pad to be "favoured" over the leading edge, and I assumed its purpose was to eliminate any sharpness in action which might be caused by the leading edge of the pads digging in .... if you catch my drift.
My replacement EBC pads came without these backing plates, but I swapped them over.
Do you have those plates ?
I'm guessing that you do, but worth checking in the absence of owt else.

Also, don't totally rule out the possibility of a "rogue" badly machined component.
I recently had a chain adjuster block which had the wheel spindle hole machined way out of square to the side faces .. I had to bore and bush it square before the wheel spindle would go through.

Is the pedal returning nicely ?
If not, that might cause the path to the reservoir to remain closed .....?
The std operating rod (via clevis to master cyl) is zinc plated and mine corroded and caused an issue .. though my master is in a totally different orientation on my rearsets, and that didn't help.
I fitted a titanium rod and clevis.

....posted before reading your last.

[Mr Gazza]

Thank you for the offer of some machining Jeff...We have some bits of sheet shim at work for tweaking ageing woodwork machines, so I have that option if needed.

I have no plates of any kind between pad and calliper..Perhaps you have put your finger on the problem, or part of it. I just have piston to pad direct (with a bit of coppaslip.)

It certainly does look like the pads, or definately one pad has been "leading in". But curiously the locking effect that I suffered happens when the thick end of the wedge is leading (When rotated backwards).

I did clean up the actuating rod soon after I got the bike, as it had considerably corroded. I haven't re-checked it yet, but the lever action apears to be satisfactory and has the all important free play which I often check.

Still hoping that this is all down to something shag-witted that I've missed...Dammit I can't wait for the bits to arrive. I'm going to hoik it apart tonight...

[utopia]

On the backing plates though ..
I'm by no means certain that all monster rear calipers are fitted with them.
In fact they may be a rarity, I dunno, my info is limited.

Ok, so how about this ...
Firstly, your pistons are retracting too far, thus allowing too much clearance and allowing the pads room to rattle further than they normally would.
Secondly, the leading edge of the pads tends to dig in (perhaps more so without the backing plates ?) and thus the pads wear to wedge shaped, thicker at the trailing edge.
Since the trailing edge is thicker, and becomes the leading edge when the wheel is rotated backwards, this picks up first on the disc and pulls the pads so that they tip to an exaggerated position due to the excessive clearance, the result being a wedging action which locks the wheel.

Does that make any sense ?

[Mr Gazza]

It's quite easy to remove the entire hydrualic system complete with calliper, master and resevoir, without breaking any hose connections or losing any fluid.

So i have bench tested it with master and calliper in vices. It's much easier to push the pistons back when I can get to it properly and apply decent leverage...Unlike trying to do it with the calliper in my hand under the bike on 6" of free hose.
Fluid flows into the resevior when the pistons are pushed in and returns as the pistons pump out when I push an allen key into the master. Everything in that department seems to be working properly. So no need to even open the envelope from Moto Rapido, Just a matter of eating some humble pie and speaking very nicely to Craig.

I am much more interested in the pad backing plates now. I seem to recall seeing them on replacement Brembo pads, but I have never had them on my brake.
If anyone has a pair on an old set of Brembo pads I would gladly buy them.

[Mr Gazza]

Sorry didn't see your last post until after I posted Jeff!

I don't think the pistons are, or were retracting to far. My initial thoughts were that they were not pulling off enough, but I am happy that all is working well hydraulically.

This business must all be down to the sloppy bush allowing the calliper to twist and so wearing the pads unevenly.
I am a little dissappointed at only getting about 4000 miles out of a bush. Maybe I should have used something better than LM grease when I assembled it?
I am thinking in terms of fitting a grease nipple on the calliper bracket, so I can give it a pump every now and then.

It could also be that everything had got a bit gummed up with it's own dust, as it seems to work perfectly now it's all clean, albeit with no greases anywhere at the moment.

[utopia]

Originally Posted by Mr Gazza

I don't think the pistons are, or were retracting to far. My initial thoughts were that they were not pulling off enough, but I am happy that all is working well hydraulically.

Originally Posted by Mr Gazza

Strangely the pistons keep returning to the more retracted position after every lever application, where as I thought it would pump back to where it was?

Ok.
I was reading the second quote above as saying that the pistons were retracting too far.
But maybe I have the wrong end of the stick .....?

Originally Posted by Mr Gazza

I did clean up the actuating rod soon after I got the bike, as it had considerably corroded. I haven't re-checked it yet, but the lever action apears to be satisfactory and has the all important free play which I often check.

The mention of "considerable" corrosion is ringing alarm bells.
How does it look now ?
If the rod was binding somehow then, although you appeared to have free play at the lever the hydraulics may still have been under a small residual pressure, closing the path to the reservoir and so on.
Your bench test of the hydraulics has not confirmed the function of the mechanical actuation, ie lever to rod to master cyl.

Not being picky, just trying to be thorough.
And anyway, we do love a brain teaser.

[Mr Gazza]

Sorry, I did kind of contradict myself there with the retraction thing.

I initially thought that maybe they were quite tight to the disc, but in fact the wheel turned perfectly normally forwards, so it was releasing okay.
After pushing the pads back and re-assembling dry I thought there was more pad disc clearance as I could not reproduce the original fault...The wheel was nice and free in both directions.
So confusing myself a bit there too as it is all so difficult to see in there when assembled.
Everything except the wear in the bush (and pads) is checking out okay so far.

I am sure I have free play at the end of the master..The rod was clean as a whistle, as I left it, so no probs there either.

I am blaming it all on a build up of brake dust at the moment, although that might not really explain the bush wear?... Could you suggest a better grease?

It is still just possible that there is some crud in the master that would block the 'ole when mounted, as the 'ole is at the bottom...I tested it inverted with the hole at the top.
I will do a fluid change in pretty short order and re-check everything so hopefully that will shift anything in there.

[Mr Gazza]

Re-test this morning and I now have an MOT...

I spent some time cleaning the calliper properly, extending the pistons and drowning them in brake cleaner resulted in being able to push them both back easily with finger pressure.
Where I had used Coppaslip, it had blended with brake dust to form a gum that must have been resticting the pad free movement.
These organic pads do produce a lot of dust, so I am resigned to more frequent and thorough cleaning to keep things sweet.

I replaced the bushes that let the calliper float and the new ones are nice and tight, with no perceptable play at the calliper end whatsoever...The calliper was moving just over a millimeter in and out before, but it was the twist that was the killer. The bush had lasted 6232 miles and still had grease in it when I removed it..The tester suggested Graphite grease, so I will try some quite soon. He also persuaded me not to fit a grease nipple for fear of grease dropping onto the disc when it exuded.

Luke kindly looked up the fitment of backing plates on the pads when I rang him to to sort the return of my new master. He confirmed that they are not an original part and are not listed for my model.
My pads had both worn on the leading edges and as a pair were about 2mm narrower at the front than the trailing edge...It was actually a very effective brake.
I still don't understand the mechanism by which it was locking in reverse, but it was definately doing it...Just a combination of wear and muck I assume....All sorted now...

As a bonus, my new spark plugs have tranformed my Monster back to it's silky smooth self again, having gotten a bit groggy around the 3000rpm mark.

A second bonus was meeting emzedder just heading out for a ride on his lovely 250 Ducati...Great to catch up and have a nice little ride out again.
250 looking absolutely superb with new ally tank and a red racing seat that I made for emzedder's dad about forty years ago for his Honda racer. (CB72 I think?)

[Flip]

I've been reading this thread with interest so thanks for posting everything in detail.

But as I haven't got the fully floating rear brake set up and have no experience of it I didn't comment but I think I would have done as you have and taken it all apart and cleaned it all and replaced what I thought necessary.

Glad you have got it sorted!!

Now we just need Emzedder to post up some pics of his 250!!

I had a similar issue on my old 996

The master cylinder seized up resulting in slight pressure being constantly applied to the rear brake unknown to me. When slowing down for a junction it somehow sheared off the mounting bolts causing the caliper to drop into the wheel and then to make matters even worse ripped out the valve core giving me an instant flat as well as a heart attack.

Keep an eye on your rear brake