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Question:

How do I unlock motorcycle disk brake?

I have bought 1 '78 Honda CB400T and everything was fine until next morning where the front disk brake locked up. I suspect that it might have to do something with temperature change as the previous owner kept the motorcycle in his heated garage and didn't take it out for quite time and when I brought it home and left it overnight on cold weather the disk got locked up. (I live in NYC, so the weather isn't that brutal though) It has been standing for a week now. I went today to bleed the brakes but nothing helped. Any suggestions?

Answer:

In addition to what Homer said, it could be an old brake hose swelled up, the master cylinder or even moisture in the brake cylinder that has frozen. Was the old fluid clean looking or very dark brown and dirty looking? If the latter, you can bet there's moisture in the system.
The piston cups are frozen because there is oxidation behind the seals and o-rings that are making them fit too tight.Try to pry the pads away from the rotor with a screwdriver (you may have to open the bleeder). When the shop had old bikes with frozen brakes we'd tap the caliper with a large ball peen hammer. That would drive the piston cups in enough to move the bike. I don't really advise this method, except as a last resort, because of the potential for damage.
Unbolt the caliper, remove the brake pads, make sure the piston is clean and smooth with no rust, pitting or brake dust so you don't damage the seal in the caliper a copper or brass wire brush and contact cleaner will work good for this- do not scratch or gouge the piston because it will damage the seal and leak there my be a rubber dust boot over the piston you can gently roll to back before you clean the piston. If the piston is pitted or scratched then the caliper need to be rebuilt and the piston replaced. Remove the master cylinder cap so the fluid can be pushed back into the reservoir when the piston is pushed in have some rags handy fluid my spill then use a C clamp or channellock pliers to push the piston back into the caliper. pull the brake lever a few times to to push the piston back out then push the piston back into the caliper do this a few times don't over pump the brakes or the piston may pop out of the caliper. If the piston moves smoothly you can reinstall the pads bolt the caliper back on, pump up the brakes and bleed the system.

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