Originally Posted by niliraga

agree that 9 times out of 10 it's gummed up pivoting that is the cause. However I have found that the springs on cheaper calipers - especially abused ones - sometimes take on a deformation/bend where they pass through the central bolt, and thus weaken in their action.

I'm no metallurgist, but I have seen some improvement by gently straightening the spring's center section back to its original shape while getting it good and hot over a flame, then quenching it into water. This has worked to "rescue" crappy calipers several times, but who knows, i might be shortening the springs' life considerably.