Shimano BR-MT63 workings
 

So I have a Shimano BR-MT63, U style brake. I was going to put it on a Schwinn High Sierra which used to have the roller brakes. But I don't understand how they work. If the spring does not contact the frame, how do the brakes have any force to go back and forth? I can't seem to get it to do so. And what is the purpose of the crossbrace piece and the oblong piece on the right? Thank you.

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The arm's return springs are connected to an intermediate carrier that gets set, rotationally, by the arm mounting bolt and how far that carrier has been rotated from no tension. IIRC one loosens thew mounting/pivot bolt, then rotates the arm to a position that will result if spring tension and the bolt is tightened securing that spring tension. Awkard but not rocket science.

The cross brace (or "Brake Booster", a poor name for a device that reduces braking force reduction) is an attempt to keep the pivot bosses from flexing apart. One reason why this brake design was made was to be able to mount the pivot bosses on the chain stays, under and just behind the BB shell. Tight next to the sharp teeth on the rings and in the firing line of both tire spray/mud and chain lube flinging off. There are many oldtimers who have the scars on their knuckles from the ring teeth from working in such a tight area. A classic example of a good goal that was poorly positioned. This design was fairly short lived in the MtB world as better cantis and then disks came about. Andy

Ah. Thank you. I will give it another try this evening.

With that right side mounting bolt loose, that offset disc piece is still way to snug to move. I would use a regular U brake if there were the 3 hole spring mount things on the posts. Or get a set of the original roller cam brakes.

How is the spring barrel or plate held in position on these? Is it just the clamping force between the fixing bolt and the mounting boss on the frame that keeps that spring in proper position?

It appears so,