I change the washer on my shower faucets but about 3 months later they start leaking again. I turn them off with a minimum of force but to no avail. I have tried buring the seats but that doesn't help either. I use tapered washers as flat washers don't work at all. The faucets stems are about 15 years old and in good shape. I don't think the seats have been changed at all. Any sugestions?