Generally, sap flow is quite normal for a Maple tree. As temperatures rise, the sap begins to move and ooze from breaks or damage within the tissue of the tree. The sap leakage will not kill the tree, but it is unsightly.

It appears there is some damaged bark from nicks, cuts and scrapes, as well as, some small pieces of bark removed in places.  If you had broken branches from a storm, sap can also leak from that damage or pruned branches. There are also some cracks in the bark. Maples are thin-barked trees and can split in a cold winter from freeze/thaw cycles or from quick thawing of sap.

Continue to watch it now through leaf out. If the leaking turns to a milky froth or a foul-smelling slime, or you notice a collection of insects on the liquid, consider contacting your local Extension Office.

Overall, most of our trees in the metro area have been suffering a bit since the "polar vortex" occurred in November of 2014. Temperatures dropped dramatically in a short period of time which did not allow most of the trees to harden off and prepare for the winter. Because of this, trees were shocked and struggled through the 2015 growing season. Here is a helpful link to fall and winter landscape care:

http://extension.colostate.edu/docs/pubs/garden/07211.pdf

And here is a link to additional resources that assist homeowners with many yard & garden questions.

http://extension.colostate.edu/topic-areas/yard-garden/