Overhauling the master cylinder

[PasoRoo]

Hi All,

After living for a while with front brakes that were still spongy I have decided to sort them out once and for all. I took the entire system off the bike and cleaned the parts. Taking the master cylinder apart I found that the bore is slightly scored. Not much, but still visible.

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Master cylinder bore

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Is this going to be an issue or should the seals be all right to keep things working properly?

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Master cylinder parts

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I have laid out the parts I took out of the cylinder (top row) and the replacements from the Brembo kit I bought (bottom row). The old circlip was mounted over the top of the dust seal. I'm assuming that is not the way it should be?
Of course the helpful instruction sheet that comes with the kit shows slightly different parts!

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Instruction manual

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The instructions tell me to install the primary seal, which I assume is the rubber seal (nr 25), with the lip towards the interior. Does the lip mean the 'open' side?
The same goes for the secondary seal (the white plastic ring, nr 27). Is the lip the side with the larger outside diameter?

Together with the kit for the master cylinder I have new seals for the calipers and new Brembo pads. Hopefully this will get the brakes up to scratch!

Cheers,

Mike

[paso750]

[PasoRoo]

Sorry if I interrupted a reply G. I was fiddling with the images to make them fit better on the page.

Mike

[paso750]

I guess I interrupted you not vice versa I only wanted to post the modified picture so one can see what you mean. The original is a bit dark.

[PasoRoo]

Thanks G!

Can you shed some light on the questions hidden in there as well as on the picture? I'm trying to figure out if I can still use the master cylinder or whether I should get a replacement.

Cheers,

Mike

[907pasonut]

Mike, I cant help you with the re-assembly, but I'm sure if you take the master cylinder to a brake shop they can hone it out for you.
the sponginess is the rasult of air, when you reassemble the brakes, bleed through the bottom nipple with the air lock hose kit (lots of instruction videos on the net) then when you think all the air is out, pressurize the brake system by forcing the brake lever with some duct tape overnight, this will raise all bubbles to the top, make sure you keep topping up the reservoir, but dont overfill and keep cover on reservoir to stop moisture contamination...after repeating procedure try out the brakes, they should feel firm and not spongy.
watch the duco, brake fluid will affect paint surfaces...

[paso750]

I wouldn´t rebore a master cylinder. If there are scratches or scars in the cylinder replace it. If there are none clean everything thoroughly and rebuild it.
The seal no.25 has the open side showing inwards which is logic as when you pull the lever this seal is what puts pressure on the system.
Don´t know about the plastic ring but if the instructions refer to the lip I`d understand that this is the side with the high edge. I suppose it´s just a guide that centers the piston as at the front it´s the seal that does this job.
The circlip goes under the dust cap.

G.

[907pasonut]

I wouldn´t rebore a master cylinder. If there are scratches or scars in the cylinder replace it. If there are none clean everything thoroughly and rebuild it.

I would agree with the reboring, if its too badly damaged its best to buy a new item, but honing is just a resurfacing done in a circular way just to get rid of the shollow scratches, where minimum material is taken off, having said that...I know little about brakes

[Jaffa]

Thanks G!

Can you shed some light on the questions hidden in there as well as on the picture? I'm trying to figure out if I can still use the master cylinder or whether I should get a replacement.

Cheers,

Mike

Mike, happy to come and give you a hand with this if you need it.

If the seal in the master cylinder is week, the pressure build up on applying the brakes will/should force fluid past it and you would notice fluid at the lever end. I dont think that is your problem.

Could just be air in the system, but now that you have it appart, the issue is to make sure you dont get any more in there at re-assembly.

PM me if you want and I will provide you with my phone number and we can discuss.

Cheers

[PasoRoo]

Thanks for the info Jaffa! I don't have any fluid leaking from the master cylinder so that should be all right.

Mike

Also: pm sent.