125 Rear Brake - Locks up!

[Html33]

Hi all!

My missus has a CBR125 05.

The rear brake has always been solid. When you apply the smallest amount of pressure on the lever the rear brake is pretty much instant, lockup, with 8 years of experience i can just about utilize the rear brake without locking up.

In a panic situation it'll have my missus off, locking up, or even if you tap it on wet roundabout, she'll be off.

I've rebuilt the caliper as it was corroded, new seals, cleaned the inside grooves of the caliper for the seals. cleaned up the piston, cleaned up the two slider pins, cleaned the brake springs, re-assembled everything and everything moves freely.

I then bled the system of air. (for longer than i care to remember, also re-bleeding the system)

The brake was pretty much the same in terms of locking up, the grab was a lot more intense, and overall braking power has improved. But it still locks up with the smallest amount of movement on the leaver.

If i pump the piston out, i can push it back in nice and easily. If i then pump it out, the leaver travels the whole distance that it should until it nips back on to the disc, and then goes solid?

It's almost like, that on the release of the leaver the piston is not being sucked in that 2/3mm it would back into the caliper, so that when you re-apply the leaver that travel of the piston is the travel of the leaver...

But as it's not returning back in, it's just nipping the disc straight away and locking up


I hope i have explained this well enough, there's loads of topics about this bike, and caliper being sized but cannot find any quite like this

Using original lines, and have not touched the master cylinder.
New straight Disc, new seals.


I've rebuilt 8 or so different brakes, both front and back and this is just not right.


Any ideas would be highly appreciated!
Thanks!!

[Howling TerrorOutOfOffice]

I'm a user of the rear brake too

From your description it sounds like you have a perfectly operating rear brake, as in the piston is sat against the pad ready for action. So long as you can still spin the wheel by hand without it binding that is.

You haven't said whether the wheel unlocks itself when you let off the lever. If it does let off then I suggest adjusting the lever so it points further down and using less pressure.
Weight transfer makes the rear go lighter so quite easy to lock the rear on a small light bike.

...and if it locks and stays locked then thats usually master cylinder, warped disc or simply crud behind the seal stopping the piston from relaxing back into position.
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[Html33]

It does operate perfectly, that's the thing..
Pads are resting against the disc, and freewheels with no rubbing (everything is lined up, chain etc)

Apply the brake, wheel stops, release brake and wheel spins freely again...Both riding and on the stand.

The response is as you'd expect.

I'll adjust the lever as it is a little bit high, but im pretty sure I've had it lower before and it was the same, but ill get back to you on that....


With all my other bikes I've had you could keep applying the rear brake, and really control and feel it before it was going to bite/lockup.
(RS50,NSR125,CBR400,CBR600,ZX6R,ER5...and a few[/size]

With this it really is on or off....unless you are really careful, but one slip and it'll have you off.

I'll adjust the lever and have a play tomorrow and perahps get a video for you.

[skatefreak]

It sounds as though you could do with having a little bit of air in the system!
Take the edge off and make it a little more progressive as such...

[MCN]

You might try a less grabby brake pad material. ???
Or adjust the lever's angle of dangle as suggested if possible.

A rear brake locks the wheel if you fart against the lever. Principle of moments. Even two up all the weight rotates around the center of gravity onto the front tyre when you slow down.

The rear brake is practically useless for 'normal' braking. Mine still has the polythene bag over the pedal from the factory. (only kidding)
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Html33]

[MCN]

It's not usual to 'hear the brakes are too efficient'.

I think develop a method of using the rear or not use the rear.

I don't use the rear unless my right hand is rolling a cigarette and I need to shave off some speed for a corner.

I ride six bikes three have rear disc three have rear drums I can lock all six rear brakes. I know this so only use fairy power to push the pedal.
Racers never stamp on the back brake they only feather it a little.


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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Pete.]

Don't deliberately put air in your brakes ffs. Change the brake pads for a less aggressive type.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Ariel Badger]

[Html33]

I've ridden, and owned lots of various bikes in the past too, all ranging in CC size and weight.

Never felt a brake like it.

I think everyone really under estimates how little movement there is, it really is on or off...

As i said above with lots of experience i can use it fine, but have to be careful. With the missus on the bike with little experience, who is still learning to get the feel of a brake, its a mixture of disaster.

Took the bike down to a garage, they agreed that something was not right and after chatting the only thing they could offer was to take it in to have a look.


Think im just going to fit braided lines, some shitter non EBC pads, and rebuild the caliper, and refit everything...


Ill get back to you all!

[arry]

[Ariel Badger]

Bradded hoses will just make it work even better!
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[Vincent]

OP.....are all the parts, esp master cylinder and brake hoses original? Just wondering if a bigger master cylinder has found it's way onto there somehow Also, are any brake shims used positioned correctly?
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[chris-red]

I'd be looking at the master cylinder too. Just an Idea is it possible a linkage is wrong/put in wrong way so that is gives more leverage?
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Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
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[Robby]

Did it go for many years without brake fluid change before you did all of the work? I would suspect the master cylinder to be a bit clogged with sludge if so, which could explain the problem.

A master cylinder needs to allow fluid to flow in two directions - pushing it towards the piston under high pressure to make braking happen, and letting fluid flow back towards the reservoir under very little pressure to let the brakes off. If one of the tiny little holes is full of sludge, very little fluid will return. Enough to release the high pressure, but not enough to release all of the pressure.

The other possibility is a broken spring in the master cylinder, or the adjustment screw on the pedal holding the brake slightly on at all times.

[Html33]

@ Vincent

All looks original, fitment and sizing all looks right, examined other images

The metal shaft that the lever rod pushes is pretty rusty so can't imagine it looking much nice on the other side of the bung into the master cyl....

No Shims, Piston to pads (ill put a picture up soon)

@ Chris-red

Linkage looks like it can only go one way, and when you push the piston in to release the pressure the lever operates how you'd expect (just no presure of course)


@Robby

You could be on to something there mate.
It had been stood for a while, and when i first got it 18months ago, the resovior was empty but the brake had pressure, drained and re-bled the system, and it worked but had the issue and then 6 months later rebuilt the entire brake and it was better response, but still the same.


I'm gunna replace the master cylinder as i think you might be right and they aren't exactly expensive...


You're exactly right Robby when you say

and letting fluid flow back towards the reservoir under very little pressure to let the brakes off. If one of the tiny little holes is full of sludge, very little fluid will return. Enough to release the high pressure, but not enough to release all of the pressure.


I think you've nailed it!!!


Adjusted the leaver last night to it's lowest and highest position and was exactly the same.

Will get back to you all!

[Html33]

Attached the diagrams
Such a simple brake lol


https://s29.postimg.org/6r6qr5dh3/F_1100.jpg
image upload no compression

https://s16.postimg.org/jxdkrzd05/F_1200.jpg
imgurl

[Raffles]

[chris-red]

You can rebuilt master cylinders with new seals, they don't need to just be replaced.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.

[stinkwheel]

David Silvers are punting out pattern Chinese master cylinders for a tenner less than a rebuild kit.

Not amazing, but work ok. Better than an old, rusty one IMO. Got one on my VFR.

https://www.davidsilverspares.co.uk/CBR125RS5-2005/part_160546/
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[kramdra]

125's do not need sintered pads, especially on the rear.. First try GG pads, they give much better feel. Brand also makes a huge difference, cant go wrong with the standard Nissin pads on a CBR6, but I suspect the standard for a cbr125 could be anything

Strip and clean master cylinder as well, you probably wont need new seals/rebuild kit if you are careful with them.

[talkToTheHat]

Change pad material. Carbon ceramic or organic. Goldfren AD compound is cheap and should tame a grabby brake. Or try honda oem?
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[skatefreak]

5 pond on Robby!