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Discussion Starter #1
1982 Honda CB900C

I have a leak between the front brake fluid reservoir and the master cylinder. It started after I had the front calipers worked on by the local Honda dealer. They made a real mess of the bike, I won't get into it but I won't go back there.

I replaced the o-ring between the reservoir and master cylinder. It only leaks when I ride it. No leaks when it sits or when I squeeze the brake lever. I suspect that when the fluid heats up it expands and causes the leak at the o-ring. The small ports in the reservoir cover look clear.

I'm stumped.

Pete
 

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An oring can leak if overtightened and smashed flat. The only other thing I can think of is damage to the mating surfaces.
 

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Thanks Dodsfall. I believe you're right about the mating surfaces. I took a good look at the groove in the master cylinder. It has quite a few scratches in it. The groove is tiny, as you know, and I'm trying to figure out a way to somehow polish it. I'd hate to have to buy a new master cylinder just because of this.
 

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Master cylinder

You can split a chop stick and insert a fine piece of sand paper. Some guys turn it with a drill, but by hand seems to be less dangerous. I will be doing this soon on my 80 XS1100.
That o ring under the reservoir usually does not leak, but the top cap can. Both will require a careful study. Too much fluid could also cause a problem. Mind you, I seem to do that often.
The reservoir itself may have been misshaped or damaged.

Unkle Crusty*
 
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