London-based Brevan Howard Asset Management has pulled off an incredibly difficult trick in the hedge fund industry: maintaining solid performance in its flagship fund as the firm's assets have swelled to a remarkable $40 billion. Brevan Howard deploys those assets across a variety of strategies, but the bulk of the money is invested in the firm's flagship fund, the global macro-focused Brevan Howard Master Fund, with $27.48 billion in assets. The fund, which combines relative value and directional strategies, has produced strong performance since its launch in 2003 and has never had a losing year. It most notably enjoyed winning years in 2008 and 2011…