Leaky Master Cylinder Piston

F8GS · Feb 17, 2020

The 12mm master cylinder has started weeping on my G/S where the lever touches and compressed the piston. Its weeping fluid into the hand guard whilst on right lock..

Can anyone recommend a replacement piston (original BMW ones are around £60) or a decent aftermarket option?

I will strip it down to see if there is anything obvious before buying. Thanks in advance.

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Beemerboff · Feb 18, 2020

It is a brake, which is a safety issue, so just spend the money and get it fixed.
Brembo made a 11mm cylinder which Guzzi used, so a Guzzi dealer may be able to supply you one, and you will have the bonus of a noticeably more powerful brake..
Never had any luck with putting new seals in a worn out old cylinder, may work short term but cheaper to fix it right first time.
Others experience may vary here, believe who you will!

F8GS · Feb 18, 2020

Interesting regarding the 11mm. What’s your thoughts on this size piston with the 320mm disc. Just fitted one. Would it give good results?

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Beemerboff · Feb 18, 2020

I have a 320 mm floating wave disk, OEM caliper, EBC HH pads and 11mm M/C and it works much better than stock, but it is still nothing special, compared with current systems on new bikes.
There is still the problem with the perch and the short lever - it looks like the ratio in the lever itself is around 5 or 6 to one, modern radial M/C's have a leverage ratio which is adjustable between 17 and 19.
FWIW Brembo still offer a M/C assembly with short levers similar to those fitted to Airheads, and they only come with a 11mm M/C.

GSSTEVE · Feb 18, 2020

These are available, bought one for my g/s, top quality.

https://shop.siebenrock.com/Drive-t...:35474.html?XTCsid=me6p0os0475asvlsbgnls1qed2

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r100gser · Feb 18, 2020

Have a look at this repair kit, or it might be worth looking if brembo do a more specific repair kit. eBay item no 352653887194

Or have a look at eBay item 303210098328 and as you brake to miss a car think of Dirty Harry saying “ do you feel lucky, well do ya”.

....ian

F8GS · Feb 19, 2020

Thanks folks for the advice and replies

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abrasion · Feb 19, 2020

There is a company that will bore out the cylinder and fit it with a stainless steel sleeve.

Leaky Master Cylinder Piston

F8GS

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Beemerboff

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F8GS

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Beemerboff

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r100gser

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F8GS

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abrasion

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