Rear Brake Dragging

[gonzodog]

My son's Yamaha 1992 WR200 trail bike has recently developed a problem with the rear brake.

The caliper and pads won't spring back from the disk after applying the brakes. This causes the pads to drag on the disk and slows the bike, as well as it has made the rear brake extremely sesitive. Only a few ounces of pressure at the pedal will lock the rear brake. Naturally this makes the bike a little hard to ride in slippery condition.

I have cleaned the rear caliper throughly, and have noticed that the slider pins are corroded. I cleaned them up with a wire brush and lubed them thoroughly so the caliper will slide on them easily. I also replaced the brake fluid and bled the brakes. The pedal is hard, no sponginess.

I am wondering if the problem isn't in the master cylinder and that somehow fluid is not flowing back up into the master cylinder after the brakes are applied, therefore leaving the brake semi-engaged.

Does anyone have any suggestions?

Thanks for your help.

 

[TWRT]

when you say you cleaned them, did you work the calipers in and out? They may be 'stiff' or semi-frozen and tend not to want to retract.

 

[TWRT]

umm, I meant work the piston in and out.

 

[David Trustrum]

cut & paste, all good:

Okay first things first. Block the bike up & spin the wheel. Take the calliper off & do it again to check the difference so you have a feel for any other problems & what the drag of the chain is like (you can take the chain off if you like). How much pedal play is there? If misadjusted the master cylinder can’t return fully. Back it off so there is a lot of play as a test.

Check the fluid reservoir cap vent is clean.

Now I would be tempted to pump the pistons out of the calliper & clean them up + check that there is no crud behind them stopping the pistons going back fully. The pad pins may need replacing. The sliding shafts need greasing with copper hi temp stuff. My calliper was so sloppy it twisted a bit so I bushed on of those pins a bit.

 

[gonzodog]

OK, I took the caliper apart last nite and found a little corrosion under the first seal - the thinner of the two. I pulled the seals, washed the caliper in soapy water, and then heated it gently with a propane torch to dry it and turn the aluminum corrosion white. It then scrapped out easilywith a small wire pick.

After reassembly and bleeding the caliper is much better. There is still a little drag, but just slightly more than my 02 KTM.

My observation is that the corrosion had pushed the seal against the piston causing it to restrict the piston's movement.

Anyway, it is much better. Cost = nothing, just a few minutes of my time.

Thanks for all your comments.