A few things may be going on. As you mention, very overgrown trees won’t fruit well, so getting them back in shape is key. This might take a few years, depending on how much work it is, but get started anytime and then plan to have an annual pruning regimen in late winter/earliest spring. Always remove all vertical shoots ("water sprouts") and any crossed or inward-facing branches that seem like they’re out of place. Dead, damaged, diseased always goes, too. The idea is to open them up (especially in the center) to more light. Another issue, however, no matter how your prune: many of the old varieties that were planted years ago are inclined toward being biennial or alternate bearers, that is, they fruit heavily only every other year, even under the best conditions. That’s the case with my six remaining old trees, which I have been keeping in shape now for 20 years. My favorite of them is pictured here, and this year will bear little or no fruit after a heavy crop in 2007.