Originally Posted by
miamijim
The RD spring is whipped.
Keith, with NO chain installed does the cage 'snap back' when its rotated? It should take a fair amount of effort to rotate the cage by hand and it should easily, if not somewhat forcefully, spring back so that the cage stop is firmly against the derailleur body.
This is a good bet. That is quite a hefty spring in there, and on a Pat '75 RD, could have seen better days.