Certified GKH Gardening Expert
These can leak sap for several reasons. The most common is damage to the tree, and insects. You will know if it is aphids, as there will be old exoskeletons everywhere. I would look for any holes in the tree, as it could be something boring into your tree. Woodpeckers can even cause this to happen.
This link will help you find your extension service in the area. You can take samples to them and they will gladly identify the issue for you: https://www.gardeningknowhow.com/extension-search/