Our property has a dalf-dozen old lilac trees on it -- most are twenty to thirty feet high. They've been having problems with entire limbs going dead and falling off -- one or two a winter, across all the trees. (We live in western Oregon -- winters here are not especially harsh, although we do get some strong, steady wind.)
I looked through your information on pruning lilacs, but it didn't seem to apply -- there aren't multiple stems to cut back down to the ground, there are single trunks of six-inch diameter.
How should we prune these trees? Should we seal where these arm-sized limbs have come off? Is there something else we should be doing?
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Sanguinity
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The Helpful Gardener
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I think that you have to gamble that these would sucker if you cut them all the way back to the ground. I think my article DOES apply to your lilacs; you just don't like what I'm saying much 
These lilacs have suckered before and the suckers were pruned instead of the old growth; now if you want to grow this plant as a flowering shrub you need to bite the bullet and go for it (you can continue to grow them as trees but as you can see they aren't very good ones)...
When the plants are as old as you say it's often a junk shoot as to whether they'd survive such a brutal pruning; I'd do it in fall after the leaves had dropped but it's a gamble either way and I don't want to make the call...
These lilacs have suckered before and the suckers were pruned instead of the old growth; now if you want to grow this plant as a flowering shrub you need to bite the bullet and go for it (you can continue to grow them as trees but as you can see they aren't very good ones)...When the plants are as old as you say it's often a junk shoot as to whether they'd survive such a brutal pruning; I'd do it in fall after the leaves had dropped but it's a gamble either way and I don't want to make the call...