Master cylinder issue

kramdra · World Chat Champion Joined: 28 Oct 2010 Karma :

Strange issue with my chinese 125, master cylinder has always felt grabby, as if it had dirt inside. When I got it, the discs were also warped, relpaced them and the entire front brake system (pre assembled Laughing

).

I stripped master cylinder, found loose paint, dirt and rust. I had a spare chinese one, I believe it was the same issue.. but clean inside.

I have just replaced with a used Nissin master cylinder from a CBR6 F3. I stripped and cleaned, but it was in good usable condition..

Im getting the same issue, not nearly as bad. Could fluid contamination cause this? could it be somewhere else?

I reverse filled (and flushed lots of extra fluid) with syringe in callipers and sucked the old fluid out from reservoir. Perhaps I should have drained it first..

It is rideable, should pass MOT as is. I could strip and clean everything again, but would prefer to know the cause first.

CaNsA · Super Spammer Joined: 02 Jan 2008 Karma :

Put bike on side stand, full left lock, watch the bubbles float up from the m/c.

You may need to bleed the lines again, but make sure the bike is on the sidestand and the bars are on full left lock.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

I would expect the CBR master cylinder to have a FAR larger bore, and even if you get it to operate smoothly you will land up with having virtually no lever travel, lots of lever pressure and a very dead feel.

However the problem you are talking about I would normally expect from a master cylinder issue. I take it that it is a sliding caliper? If so check it slides smoothly on the pins.

All the best

K
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Rogerborg · nimbA Joined: 26 Oct 2010 Karma :

You haven't replaced the hose? Could be crud stuck in there.

https://www.youtube.com/watch?v=xlmZH6SKUCc
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kramdra · World Chat Champion Joined: 28 Oct 2010 Karma :

Snod Blatter · Crazy Courier Joined: 21 Nov 2014 Karma :

Since you have fitted a used item, have you tried cleaning and regreasing the lever pivot bolt and any other surface the lever travels on? It always amazes me how much this can transform the feel of the lever.
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL