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Old 13-10-18, 01:45 PM   #11
Bibio
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Default Re: Rear brake still sticking after servicing.

over tightening of the rez cap, this can cause a vacuum.

too much RRG when assembling the piston and seals. this can actually cause stiction.

resting your toes on the brake lever. this is very common and people dont know they are doing it. solution is to move your toes outwards while riding, or put your toes on the pegs. another is to move the lever down further by adjusting the MC rod.
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Old 13-10-18, 04:38 PM   #12
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Default Re: Rear brake still sticking after servicing.

Quote:
Originally Posted by Bibio View Post
over tightening of the rez cap, this can cause a vacuum.

I wasn't aware of this. I'll check it when reassembled.


Quote:
Originally Posted by Bibio View Post

too much RRG when assembling the piston and seals. this can actually cause stiction.

I might be guilty of this! Again. will check when I reassemble.. looks like I'm popping the piston out again first too.


Quote:
Originally Posted by Bibio View Post
resting your toes on the brake lever

I've been very good and disciplined with this for a while. Balls of both feet on pegs unless moving feet forwards to shift or brake. Concious of doing this since trying to work out the dragging brake issue a while ago, and it's good practice anyway.


Thanks for all suggestions.


I've stripped and inspected the master cyclinder and it's really clean, all parts good and no problems appeared. Back together now and will reassemble the system and test again.


One last (probably silly) question.. The reservoir connection on the MC has two holes on the MC wall, one larger and one very small, almost pin sized. This is correct yes? I haven't got one that hasn't been machined correctly and the pinhole sized one should be larger?
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Old 13-10-18, 08:02 PM   #13
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Default Re: Rear brake still sticking after servicing.

All back together. Popped piston again. Refitted it and bled again. Brake lever going rock hard with little movement and brake binding on again. Not as bad as before. There's a tiny bit of movement of the caliper but it's still dragging.


Defeated for today.
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Old 13-10-18, 08:21 PM   #14
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Default Re: Rear brake still sticking after servicing.

Are both pads dragging, or only one? Are the pads striking the disc square?

If the pads are dragging too much they get the disc so hit the heat then starts causing the problem to get worse. After a mile or so it's almost full braking. (Happened on my SY years ago. On that occasion it was a clear case of seals.
Also once found the caliper carrier bent on my SK5. That was similar result.
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Old 13-10-18, 08:45 PM   #15
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Default Re: Rear brake still sticking after servicing.

Both pads dragging. Caliper and disc are both warm after riding. The seals are new. The seals I removed before these were almost new too. Piston is new. Hel brake line is nearly new. Master cyclinder is all moving nicely. The piston just doesn't get pulled back in after brake is applied. The seals aren't pulling it back away from the disc.
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Old 13-10-18, 08:53 PM   #16
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Default Rear brake still sticking after servicing.

How warm is 'warm?'

Is it ouch to touch, or a hold it and it seems warm? Or is it that your finger prints stick to the disc, fry and don't come off hot?

Is the disc turning blue?
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Old 13-10-18, 09:11 PM   #17
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Default Re: Rear brake still sticking after servicing.

Not too hot to touch for a short time. But warm even without using the rear brake. Both front calipers and rotors are cold even though they're the ones that get used.


Disc has not discoloured.
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Old 14-10-18, 10:37 AM   #18
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Default Re: Rear brake still sticking after servicing.

Front discs get a lot more cooling air than the rear does. All disc brakes drag a bit, there is nothing to pull the pads back except a bit of flex in the seals. With drum brakes there were big springs to pull shoes away from drum, with discs all you are doing is releasing pressure - if the disc is perfectly flat and true there is nothing to push the pads away (if discs are not true the bumps will knock the pistons back, but not good for braking). As long as you can comfortably touch and hold the discs they are below 50deg C - not enough to damage anything - are your rear wheel bearings OK, is the heat coming from them ? Can you get rear wheel off the ground and spin it and listen for noise ?
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Old 14-10-18, 10:39 AM   #19
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Default Re: Rear brake still sticking after servicing.

Too much fluid in the system? Is the brake lever pivot sticking?

Also what happens if you manually pull the lever up so it is definitely not applying the brake then ride it without ever using the rear brake. Does it come on by itself or only stay on after use?Try bungying the lever up while testing this to prevent it going on.Does the wheel turn freely while up on the stand?
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Old 14-10-18, 11:11 AM   #20
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Default Re: Rear brake still sticking after servicing.

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... there is nothing to pull the pads back except a bit of flex in the seals. Can you get rear wheel off the ground and spin it and listen for noise ?

I think this is part of the issue. I don't think the seals are flexing with the piston. Possibly the seals I've bought, or maybe an issue with the caliper I got a while ago to replace the original that someone had cross threaded the mounting bolt on. I'm having a go at overhauling the original caliper and have managed to correct the threads, fitting the bolt from the opposite side and recutting the threads. Hard work as had to be done off the bike threading bolt from the inside but seems to have worked. The original caliper was a lot better condition than the replacement apart from the cross threading problem, so here's hoping if I use it again the issue will be solved. Having another afternoon in the garage today it looks like.


I have the bike on an Abba stand and wheel spins but lots of drag and no visible movement in the piston retracting. Not even a fraction of a mm.
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