Question
Hi. My weeping cherry is over 20 years old (Central NJ). In the spring there was thick sap on the trunk of the tree. I pruned the branches in the spring. The leaves are now turning greenish-yellow and some of the branches are dead with brown leaves. We have had a snowy winter and a wet spring. I don't know if this is due to a bacterial disease or damage from a leafhopper etc. Please advise me on how I can save my tree.

Answer
There are several different things that can make a cherry bleed sap.  The most common cause are peachtree borers.  If it is borers, they are usually close to the ground and you will see wood/sawdust mixed in with the sap.  Below are links to two factsheets that show pictures of the insect and give information about control.  Borers would be good news because there are treatment methods.
http://extension.usu.edu/files/publications/factsheet/greater-peachtree-borers07...
http://ag.udel.edu/extension/horticulture/pdf/ent/ent-14.pdf

If the problem does not appear to be borers, it may be due to a canker pathogen.  There are three main pathogens that cause gumming on cherry.  They are bacterial canker (Pseudomonas syringae), Botryosphaeria canker, and Cytospora canker.  Unfortunately, since the symptoms have reached your main trunk you cannot prune this disease out at this point.  Your best advice is to promote health and vigor by watering during periods of drought and maintaining proper fertility.  If the tree continues to decline rapidly, you may want to have it removed and the wood discarded.  I'm not sure what other trees you have in your landscape, but some of these pathogens can spread by windborne spores.  It will be sad to lose this tree, but devasting if the problem spread to other trees.  Botryosphaeria in particular has a relatively wide host range and enters trees through wound sites.  

The last possibility I can think of is winter injury.  If you had a harsh winter, sometimes the tree wood can be damaged and cause cracks.  The tree will exude sap through the cracks.  If this were the problem, I would not expect the tree to start turning brown after leafing out unless something else was involved.  Unfortunately, I think your tree may have a more serious problem.

You may want to have your actual tree looked at by an arborist in your area.  You could also contact your local county extension office and one of the extension educators may have more information for you about specific problems on weeping cherry in your area.  To find your local extension office, you can visit the following website:
http://njaes.rutgers.edu/county/

Good luck!