I have let unused 30 lb mono sit in a closed tackle box in a cool basement and suspected it may not be as strong as new mono and it was not. The old mono broke at the knot fairly easily while the new mono, same type, berkley big game broke the swivel I was using to test with. Take a portion of the old line and tie a swivel to each end. Put 1 end in a vice and grab the other with pliers, for me it was not even close, new line was substantially stronger.... wear eye protection when doing this