Why is my ornamental cherry bleeding and losing leaves?

The decline in leaves that you are experiencing is likely related to an organism that is inhibiting the flow of nutrient and water through the vascular system of the tree. Based on the observation of bleeding sap, it sounds like you have either a bacterial/fungal canker, such as Phytophthora or a boring insect, such as peach tree borer. Cherry trees are very susceptible to these types of pests. I would recommend having someone inspect the tree to determine what treatment course would best address you problems. Decline in leaves is typically a result of numerous stress factors which would be best identified in person.