Weeping cherries are hosts to a number of diseases, fungal as well as viral, along with insect pests.  The very best defense for your tree, and all trees for that matter, is to keep the tree healthy.  Trees do a pretty good job of defending themselves when they are healthy.  Your tree is obviously not in good health currently however.  If your seeing scale or other insect pests in the tree, it could very well be secondary.  Often the tree can be afflicted by both insects and disease, the second taking advantage of the trees diminished state.  A positive diagnosis of everything going on in your tree is the first requirement before any treatment can be considered and the tree really needs to be evaluated by a professional. Recommendation is an evaluation by a Certified Arborist, specialists who specialize in everything woody.  You can find one or more in your zip code at www.treesaregood.com  

Arborist  evaluations are generally not expensive and you could then decide how to go forward based on professional information.  I've done this myself on several occasions and learned quite a lot in the process.  Sooner rather than later as time is not on your tree's side. 

Good luck!