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Front Master Cylinder Leaky Ugly Mess
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radioguylogs
Posted 2018-11-15 11:45 AM (#154076)
Subject: Front Master Cylinder Leaky Ugly Mess



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Location: Brighton, MI
I know it happens to others, because it has happens on my XS bikes.

The Front Master Cylinder develops a slow leak between the nylon bowl and the aluminum base.

At first, a white crust appears around the perimeter of the seam. Eventually, the paint peels around the outside of the whole base, and ugly white and gray corrosion takes over. Big chunks of yellow- white crud accumulate. You hope nobody sees your bike in this condition.

Sound familiar?

It's not covered in any service manual or exploded part views, but it's not hard to fix.

Borrow Mom's turkey baster or use some paper towels to empty the brake fluid out of the reservoir.

With the four lid screws still removed, and the diaphragm still out, rotate the nylon bowl until it breaks loose from the base. Get a knife or something thin to start prying the nylon bowl up form the base. As soon as you can work it in there, get your biggest screw driver inserted and carefully pry the bowl up from the base. Don't bugger it, be patient.

You will find the bowl is sealed to the base with a big O-ring. You will find that brake fluid has crystallized around the O-ring to the point the O-ring is no longer effective.....just like the brake calipers you already rebuilt.

Clean out the crystallized brake fluid crud and aluminum oxidation using a small screw driver and Dremel wheel.

Match the O-Ring at your local hardware store. (It's not on any parts diagrams, so this seems to be the best way). I chose metric-sized nitrile this time.

Now is an OK time to install a rebuild kit in the cylinder if you think it needs it. I find that if you are curious, and you try to remove the rubber boot around the plunger, it will almost always tear before you get it off, and now you are committed to a rebuild kit because you can't buy that boot by itself.

Clean and paint the base with semi-gloss or satin black enamel.

In a few days, after the paint cures, reassemble the Master Cylinder and marvel why you didn't do this 5 years ago.

I just did this again on one of my XS750SF's. I wish I had taken pictures to help the next guy, but I was done before I got the ambition.

Hope it helps,

-Mike
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