FBinNY 

From your description I assume you have a plain tubular bridge that lacks a brazed on flat surface for the brake bolt to rest on. (see the photo in the prior post)

This is a lousy system which depends on two half round washers to provide a mounting surface, and also avoid crushing the tube hen tightened. Rotation under brake load resisted by friction alone and can be inadequate to the task. Trying to make a tighter fit of the bolt in the hole won't help because the torque is too high.

Here's an easy cure. Use coarse lapping compound - grit in grease - on the half round to tube mating surface. When the bolt is tightened the grit will bite into both then bridge and washer making a secure mechanical interlock to keep the assembly stable. Lapping compound isn't cheap, but you only need a dab, so beg some from the local auto engine rebuilder, paying with a cold beer.

Alternately, you can glue adhesive sandpaper, or emery cloth to the mating area of the half round washers to do the same thing. Or you can use super glue or a Loctite bearing retaining compound to bond the washers in place on the tube (you should sand away the paint first for a good bond). Be sure to position, tighten and allow to cure before riding, and do not get any adhesive on the brake bolt itself.

Lastly you could use JB Weld and a stainless steel washer to build up a flat plate similar to what's on better bikes for a permanent job.