I take it we are talking 'plastic' pulleys? the older pulleys common in the late 50's onwards I believe were made of a 'Celluloid' type material...the same type of stuff that Gi Joe (Action Man) was made from, even NOS boxed figures sometimes when removed from their boxes have fallen apart or have serious cracks were stress was applied during manufacture, the material does degrade with age but much Will depend on how the item is made/stored and ultimately used. I wouldn't have thought a pulley would be under enough 'stress' to fail.
Steve Chambers ,Chesterfield, England.