The best time to prune trees generally is in late winter or early spring (March or April) before the leaves bud out. However, since maple trees are among those which will bleed a lot of sap if pruned that early, the next opportunity is during the summer, probably mid to late July. . Examples of trees that bleed excessively are maple, willow, birch, walnut, beech, hornbeam, elm, and yellowwood.

Although the research says that this bleeding sap is not harmful, it is unsightly and if the trees are sugar maples, the sap may be sticky and attract insects. In mid to late July the sap is no longer running, and the tree has received the bulk of its nutrition from the foliage. You didn't specify if your trees are sugar maples or another variety. Attached are several articles about pruning in general, which also mention maple trees.

The best time to prune these “bleeder” trees, as mentioned above, is in early or mid summer after the leaves have hardened or turned dark green. The sap will still run, but not to the point of depleting moisture and sugar from the tree. Since it is not a close system, like a mammal, the tree will not “bleed to death”. Late summer pruning is also discouraged because, although there will be no bleeding immediately following pruning, the non- healed cuts will begin to bleed as soon as sap begins to flow in spring.

Whatever pruning you decide to do, never remove more that 25% of 10-year old trees at one time.

Removing the lower limbs of the tree up high enough will send more water and nutrients into the crown of the tree where primary growth occurs.

Here are a few links that may be helpful to you:

Late Winter Recommended for Pruning Chores - Purdue.pdf

Pruning Deciduous Trees – Wisconsin Horticulture.pdf

Tips on tree and shrub pruning - MSU Extension.pdf

Trees and Threes_ Prune Now to Keep Trees Healthy – Think IPM Blog.pdf