1977 GS750 rear brake dragging

[850GT_Rider]

The reservoir cap / boot assembly has nothing to do with the problem OP has described.

Nessism is on the right track....the return port is either blocked by debris...... or.... the piston assembly is not retracting completely thus allowing the secondary cup to block the return port. Thinking that because M cylinder was removed and rebuilt, and IF for sure the return port was clear, does the master cylinder piston / cup assembly bottom out properly against the washer / internal snap ring when brake pedal released? Is there an adjustment to be made on the push rod itself? If the piston was part of the rebuild kit and if it was replaced, was it the same in every dimension as the original?

EXACTLY

In the first post, he said he "replaced and rebuilt" the master cylinder. (Those are two different things, but concept is same) If the pedal and travel were not properly adjusted, and the rod is "too long" as he stated, there is not enough free travel at the start of the stroke. The piston will not retract far enough to clear the compensating port, the calipers will not fully retract, and the brakes will overheat. May not happen real bad on short trips, but take a longer one and......

Don't ask me how I know this .....

"Experience" is the name we give to our list of mistakes. Sometimes, I'm really "experienced".

[Steve]

"Experience" is the name we give to our list of mistakes. Sometimes, I'm really "experienced".

I have heard that expressed as: "Experience is recognizing a mistake when you do it again."

.

[850GT_Rider]

Guilty as charged....