Wrong voltages on underground cable
It sounds like you answered your question. With no voltage on b2 and reading 120v b1-b2 it looks like b2 has opened and is in contact with earth since there was 240 at the breaker panel.
 Some tracers may get confused with 240v because the 1 hot continues, I have seen this and disconnected the good hot then finding the break was fairly quick as the signal disappears at the break. Make sure to open the disconnect since you know the break is prior to that and the signal won't be able to return through the pump windings, makes for a clean signal.