One thing that could cause the problem is using a belt with the wrong cross section. That means, if you would cut the belt,maybe an old one, and look at how the cut end fits into the pulley sheave, you might find that is the problem. As you leave the clutch pedal out, the normal shaped belt will nestle down into the sheave, and not hurt the lower flange, but a mis-matched belt will tend to squeeze the lower or upper flange of the pulley out of shape and eventually cause it to break off. It doesn't seem like that could be a problem, but it could be one.