(Reuters) - Warren Buffett's Berkshire Hathaway is being sued by the billionaire Haslam family, which accused the conglomerate of using improper accounting to devalue its remaining 20% stake in Pilot Travel Centers, the largest U.S. truck stop operator. The family, including Cleveland Browns football team owner Jimmy Haslam, sold Berkshire a 38.6% Pilot stake for $2.8 billion in 2017 and another 41.4% stake for $8.2 billion in January. It said it has a right to sell the remainder under the same valuation methods on Jan. 1, 2024.