Rear disc dragging

[gray am I]

I took my 79 special out for a run after a long time not riding and i noticed that the rear disc is dragging, and it was quite hot ,unlike the two discs up front. left it for a week on the center stand and it had freed up, applied the rear brake lightly and it was still rotating freely but wnen I applied more pressure it is creating drag on the disc. I need help diagnosing ,do i need to disassemble the piston, clean and reassemble or could it be something else?

 

[Its been a long time]

Check the caliper is floating on the sleeve and bolt, parts 7 and 8 below. Otherwise, yes, probably sticky caliper piston.

 

[mike in idaho]

Crack the bleeder screw loose briefly next time it's dragging. If it releases, with the bleed screw open, that means the master cylinder piston is not retracting enough to open the relief port inside. Still dragging?, time to overhaul the caliper.

 

[Signal]

The pressure relief hole in the master cylinder may be blocked.Normally when the lever force is removed the fluid under pressure flows back into the reservoir and releases the pads.
Another thing is the piston could be encrusted with pad dust and road grime and the buildup is stopping the piston and pads moving off the disc.
I think first try giving the assemble a good wash if it's dirty. Remove the pads but leave the piston in place.
Also check that the bush (part 7 in diagram) and bolt (8?) move smoothly in the rubber seal (6?), these can rust .
The caliper moves across on the bushing as the pads wear.

 

[gray am I]

thanks for the help guys

 

[ThatXS650Guy]

That happened to mine when the rear master was locking up. A rebuild solved the problem.

 

[OHRider]

The pressure relief hole in the master cylinder may be blocked.Normally when the lever force is removed the fluid under pressure flows back into the reservoir and releases the pads.
Another thing is the piston could be encrusted with pad dust and road grime and the buildup is stopping the piston and pads moving off the disc.
I think first try giving the assemble a good wash if it's dirty. Remove the pads but leave the piston in place.
Also check that the bush (part 7 in diagram) and bolt (8?) move smoothly in the rubber seal (6?), these can rust .
The caliper moves across on the bushing as the pads wear.

The relief hole is very small (about the size of a paper clip). A good cleaning should fix it.