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Mr Gazza · 01-08-2016, 07:38 PM

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?

01-08-2016, 07:38 PM	#1
Mr Gazza Lord of the Rings Join Date: May 2013 Location: Norwich Bike: M900sie Posts: 5,810	Sureal goings on at the rear! 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? __________________ Last edited by Mr Gazza; 01-08-2016 at 07:44 PM..