The first rule of gambling is to know when to fold ’em. But after the creation of the Volcker Rule, a regulatory law meant to prevent overly risky trading, JPMorgan Chase sent lobbyists to Washington to argue for loopholes that would allow for trades much like those that led to a $2 billion loss announced by the bank Friday. Bank chief executive Jamie Dimon and other members of upper management paid regular visits to lawmakers to argue that, while they thought some parts of the rule were useful, others would hurt the bank’s ability to hedge against risk. The result, said Sen. Carl Levin, was a “big enough loophole that a Mack truck could drive right through it.”