It is likely your cherry tree's running of sap is a symptom of canker. Canker is caused by bacteria. Often a branch with canker must be removed. I would recommend contacting a local tree care company and request a consult with an arborist. The arborist can confirm what is causing this and suggest what is needed to keep the tree healthy. Below is a link with information about bacterial canker. 
https://extension.psu.edu/cherry-disease-bacterial-canker