Repairing Double Hung Sash Cords

Replace the Window, Then Replace the Stop.

now attach the sash cord from the top of the pulley to the window, reattaching it just as you took it off with the two nails. You need to ensure you have both a freely moving sash weight and that the sash weight has just enough sash rope to extend to the bottom of the sash weight canal when the window is fully extended up.  I always seem to do a little more fussing on this aspect. Again, even though you are very near the end of the job,  take your time no rush. Definitely test the window for movement before reinstalling the STOP.
put the stop trim back on. The snazziest and most straightforward way to do this is to curve the stop out, decreasing the vertical length of the stop an inch or two allowing you to fit it in very snugly back precisely where it came from. No need to create a big bow in the STOP – just an inch or two will do the trick. Use the same nail holes and same positioning to remount the stop.
Tap the finishing nails back in place along the length of the stop trim. Make sure the window is moving freely.
check that the top and bottom meeting rails are flush. you’ll often get a slightly miss-seated window, or a small piece of construction material caught underneath the window. check this again. a third time won’t hurt either.
and here we go a rehabilitated window that works and looks good. This will stop the cats from escaping and you can open and close with a single finger on your hand. A coat of paint and we’re done!