I keep breaking vacuum cleaner belts on my Sanitaire SC689 vacuum cleaner. The roller turns freely and the motor sounds OK. Anyone got any input

Belts are considered a wear and tear item that often needs to be replaced. Belts for the S689 is a very common size so you can try using different brands. If you are using just a standard black belt you might want to try clear (I've heard some people say they last longer) or you can try using the original belt from Eureka which seem to last longer than the "generic" ones.