Front disc brake locks up!

[Quinn_McLeod]

Hey guys,

I've had repeated problems with my stock '78 front single disc brake locking up on me. It starts to rub and the disc gets so hot it actually steams at a stop light. Whole bike drags and eventually once I get the bike home and in the garage, I can't even push the bike forward because the front brake is all locked up. I've got newer pads and I freshly rebuilt the front brake. This has happened twice now. I've got the manual and I've taken it apart so many times I've got the thing memorized. SUPER frustrated and want to sell the bike because I can't ride a single 1/2 hour day without the stupid thing freezing up on me.

Quinn

 

[chizler62]

Are you making sure the caliper mount slders are free? Can you move the caliper on the mounting bolts?

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[figure8]

There was just a similar thread like this. I'll try to find it but I think it was suggested as a problem with mc, not the caliper. Have you checked the condition of the master cylinder? It sounds like after you engage the brakes and let off the fluid is not going back up to the master cylinder thus keep your caliper engaged.

 

[figure8]

http://www.xs650.com/forum/showthread.php?p=413596

Read Ggggarys post.

 

[XSLeo]

Read my last post on that thread. I talk about how to unplug the bleed hole.
Leo

 

[weekendrider]

It is also possible the brake line has deteriated and a "flap" of line is preventing the fluid from returning.

 

[XSLeo]

Yes, taking the old lines off and hanging them on the wall is the best thing for you and your bike. Replace them with new.
Old lines rot from the inside. Bit of this rot float around in the fluid, some make flaps as weekender said.
Leo

 

[Brassneck]

Had a honda did the same thing...bad master cylinder...and old brake lines.

 

[weaselbeak]

While it's in a bit of a drag situation, break the bleeder loose. That releases line pressure and if the wheel now spins freely your problem is master cylinder/line related. If it's still drags, you have a problem with the caliper not returning. Just helps to know where to start.

 

[Quinn_McLeod]

While it's in a bit of a drag situation, break the bleeder loose. That releases line pressure and if the wheel now spins freely your problem is master cylinder/line related. If it's still drags, you have a problem with the caliper not returning. Just helps to know where to start.

Thanks everyone! That was helpful. I was a bit baffled since everything is new. New caliper rebuild kit, all new rubbers, new brake lines, and new master cylinder rebuild kit. Installed it and it worked fine. Then out of nowhere it started to lock up randomly. Eventually opened the master cylinder to find that the rubber flap had depressed and wasn't releasing. Thus the brake fluid couldn't make it's way back to the master cylinder. Removed the top of lid of the master and pumped the lever a few times. Fluid returned to the cylinder and I was able to put the cap back on and it ran fine. Had to do it twice to get the rubber to get back to it's original shape. Basically the master cylinder rebuild kits never come with a new rubber seal for the cap. I've got the rectangular shaped one '78.