Front brakes locking up!

[microspeedracer]

I had my front brakes lock up at the motocross track today twice. They would lock up and the brake lever wouldnt move at all. The first time I bled it off and worked normal for awhile, then it did it again. Loaded the bike up and by the time I got home they were freed up. Any ideas what the heck is going on? It didnt do this last week. Thanks for any replies in advance.

[J.B.]

Sounds like you need to rebuild the calipers and/or the MC. Rebuild kits are on Ebay for both.

[wilkin250r]

I think you got water in the lines.

Brake fluid is hygroscopic, which means it will absorb moisture from it's surroundings (including humidity in the air). It still works okay, because it's still fluid and you can't compress it, but every bit of moisture lowers the boiling point of the brake fluid.

Too much moisture, and it begins to behave like water, which will expand and build pressure when heated (even if it's not "boiling", you get evaporation from the surface, and the gas expands).

Since this heat generally comes from friction at the brake pads, the heat is in the caliper, which means the EXPANSION is in the caliper. Now you have a big problem, because more heat means more expansion, pushing the pad against the rotor harder, which just creates more friction which creates more heat.

[wilkin250r]

I had the same problem on the rear brake on a streetbike.

Just changing the fluid would fix it, in theory. But I tried that and the situation only improved, but didn't fix. Maybe it just didn't come all the way out. I didn't actually fix the problem until I pulled it all apart and cleaned every bit of fluid out.

You can try just changing the fluid, but I think you're in for a complete rebuild.

[microspeedracer]

Thankyou for the replies. Any idea where it might be pulling the moisture from? It was a stock closed system until I added longer steel braided lines when I put on my +3 a arms about 2 months ago and you can about imagine the amount of new brake fluid I went through to get them bled well.

[atvmxr]

DOT 5 is synthetic and not so prone to the gyroscopic problem...

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