Good evening, in reality it is always better to change the fly-back, but if the loss of high voltage is somewhat slight, just a slight noise to "hiss" you can use a paint that is used for this purpose, I in my locality with it an acrylic of the Delta brand, is applied throughout the body of the fly-back surpassing by a few centimeters the application to the high voltage cable, apply a hand to cure and repeat one more hand .. This process takes a few days, if He wants to do a good job.