If you know enough about Graves and Printz/Prince to have a full name of Ambrose Barbour, you must know whether the service was in County Militia or State Troops.
You do not identify the unit.
Records of State Troops would be at the VA State Library.
Records of the County Militia would be in the County Court records, which also would hold information about when or whether Levi was commissioned Captain.He could have received a Militia commission well after the Rev. War was over.
They are not listed in the Continental Army rosters.