The short answer is that, if B actually cuts the brake line, both A and B can be convicted. That would at least be a vandalism charge, and possibly attempted murder, depending on the specific circumstances.
B would be liable as the direct perpetrator, since he did the criminal act of cutting the brake line. A would be liable as an aider and abettor or an accesory because he got B to do it. (Although we usually think of an accessory as being less culpable than the direct perpetrator, that is not necessarily so, as this example demonstrates.)
That leads to the question, if B was basically forced to commit the crime, why would he still be criminally liable?
There are only limited circumstances where the law allows someone to "get away with" committing a crime. Generally, if someone is being threatened or forced to commit a crime, they would be expected to report it to the police. That's true even if reporting the crime would have dire consequences, such as the loss of a job.
There are some defenses where a person is basically forgiven for committing a crime. However, the crime must be committed to prevent something worse from happening, and it must be the only real option. Self-defense or defense of others would be such a defense.
There are also the defenses of duress and necessity. These defenses have strict requirements, and generally only permit someone to commit a crime if the crime would help prevent some bigger wrong, and if there was no other reasonable alternative.
For example, if someone has planted a bomb and threatens to blow up a school if you do not rob a bank, and you have no way to notify authorities or get away, you would probably not be liable for the bank robbery.
Being threatened with losting one's job would not qualify under any defense.
Note that in teh scenario above, A did not commit blackmail or extortion by threatening B's job to force B commit a crime like cutting a brake line. Extortion involves threatening someone to force them to give you money or property (or to get a public officer to perform an official act).
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