Leaking front master cylinder cap

Is it an aftermarket cap? Seems as though aftermarket caps tend to leak a little. Remember, brake fluid, especially Dot4 and Dot5 is very thin viscosity wise and will find even the smallest iopening to leak out from. Another problem is overfilling. You only need enough brake fluid to fill the sight glass 1/2 full when the master cylinder is sitting level (you'll need to position your handlebars and/or stand the bike up straight to level the cylinder. That's important. The 4-5 fluid isn't nearly as corrosive to painted finishes as the glycol based 2-3 fluid.

Like Flip said, you don't want to overfill it. You really shouldn't have a leak though. Carefully inspect both the cap and the master cylinder reservoir where the rubber seal makes contact. Use a bright light and possibly a magnifying glass if you have one. Look for any nicks, burrs or casting flaws. You might use a straight edge and check both the cap and the master cylinder for warping.

While not very likely, also check to see if the master cylinder is leaking out the bore and bleeding up to around the cap area.

When installing a new rubber seal make sure all surfaces are dry and don't over tighten it, causing the seal to squish out. Don't use gasket sealer. Use only the recommended brake fluid.

You can spray off the master cylinder with brake cleaner (cover up the rest of your bike so you don't get splatters on your paint and chrome) so it is completely clean and dry. Sprinkle some baby powder on your master cylinder cover and go or a short ride to make it leak just a little, and hopefully you will at least find the exact spot the leak is coming from, as the baby powder will be wet at that spot. It might help you narrow down the cause.

Thats all the ideas I have right now..

cycleheadlp said:

I have an unusual problem with a leak around the cap on my front master cylinder. I removed it to add fluid, and now it wants to seep whenever I ride it. It does not leak when sitting or if I pump the lever. It only leaks when I take it out for a ride. It is only a small amount, but the stuff is so corrosive that any amount is too much. I ordered a new rubber seal to place under the cap, but it leaks the same as the original one. Has anyone any ideas as to what would cause this? The fluid level is right, and it did not leak before.​

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I had the same problem with my new 07, (I was bleeding the brakes to get rid of spongy feel, which did not work I had to pull the pistons and lubricate them with brake fluid) they gave me another rubber seal under warrenty and it leaked too. I finally went an got some cork gasket material and cut a gasket to shadow the rubber seal and it has not leaked since. my dealer is 3 hours away so i can't take it to them for every little issue i wish I could. Good luck with it.

Fix spongy feel?

headvince,

To fix the spongy feel, I tried bleeding mine as well, to no avail... Am I understanding you correctly, that removing pistons and lubricating helped fix spongy feel? Did you do this to both the master cylinder and brake cylinders?

ratpick said:

headvince,

To fix the spongy feel, I tried bleeding mine as well, to no avail... Am I understanding you correctly, that removing pistons and lubricating helped fix spongy feel? Did you do this to both the master cylinder and brake cylinders?

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I just did it to the front brake calipers. You may want to try taking calipers off and just pushing the pistons all the way back into the caliper sometimes this will work without taking them out. I have pazzo levers and had them on setting 6 now thy are on 3 which for me is alot better.