Break, d--n you break!

[Volcano Man]

Optional Assault Resolution rule is the most important one to use in regards to assaults and since you are using it then that is good. But as Sgt Fury said, you have to do a lot of damage to a unit to both have it disrupted and max out its fatigue before it can be broken.

Other than that, a unit can break as a result of an assault but the first two prerequisits must be met first. The best thing to do though is to not wait for a unit to break, just surround and isolate the unit and then disrupt it. After that you assault the unit and you will get most of the unit to surrender since it cannot retreat.