I would like to know how to prune a very old hydrangea vine growing on the side of my garage. It was badly pruned about 6 or 7 years ago when the centre and bottom were cut way back. But the top was left intact and it is top-heavy.
It sounds like your climbing hydrangea vine has been damaged by the earlier pruning to the middle and lower parts and thus should be pruned back heavily to rejuvenate the plant. The best time to perform a major pruning is when the plant is close to coming out of dormancy in late winter or early spring. Prune back the majority of the plant, leaving three to five 3-foot-tall stalks. After a heavy pruning, avoid pruning your climbing hydrangea over the next year.
For more in depth information on pruning hydrangeas here are a couple of links that could be helpful to you: https://www.hydrangeashydrangeas.com/pruning.html