A healthy apple tree in good soil in the absence of severe pest problems or drought is likely to make more growth than it needs, and this must be controlled by maintainence pruning in order to achieve steady cropping. This can be done in summer according to the growth of the tree. Every tree, every year is different. For example, in my orchard this summer (July 2004) I have 2 groups of Egremont Russet, one 12 years old in good soil, the other 6 years old in poorer soil. The trees in better soil have set very little fruit this year and have made an enormous amount of new, thin leaders which are choking up the centre of the tree. I will be removing three quarters of these growths next week, pruning them down to half an inch to balance the tree. However, the younger trees are carrying a reasonable crop of apples and they are making a reasonable amount of extension growth. I won't need to prune them at all this summer, although I will be walking round the orchard with my (frequently cleaned and sharpened) Felco number 8 secateurs and making a few snips here and there, particularly to remove any diseased wood. (NB as of 22 August, the smaller younger Egremonts are on average carrying twice as much fruit as the bigger, older trees. I believe this is because the rootstock and the soil are in better balance, more by 'luck' than planning. Matching rootstock to soil is acknowledged to be one of the most difficult problems in planning an orchard. As a rule, rich soil, weaker stock, poorer soil, bigger stock-but MM106 is a good compromise which rarely lets you down)
The classic book on summer pruning is by the Frenchman Louis Lorette. He put a great deal of theory down in a book that's hard to find and not an easy read, but his approach changed pruning for ever. To get, and ripen, maximum apple and pear crops it is necesary to prune in summer to remove excess wood and leaves to allow more light and air into the tree to get the best crops. This is counter-intuitive but it works.
Take a look at your trees around midsummer, if there is an excess of young growth, remove some of it. Don't tip all the shoots back, leave some, remove others by snipping them down to about half an inch to an inch from their origin (hopefully a fruit spur will form there). Always make such cuts just above where a leaf arises, the bit above the pruning cut will die back to the leaf so if you cut too high you will have some dead wood which should be avoided if possible.
Additional material on summer pruning is available from both the RHS and BBC Gardeners World. Follow the links below: