For Sun Capital, the process of buying companies, seeking profits and leaving pensions unpaid is a familiar one. Over the past 10 years, it has taken five companies into bankruptcy while leaving behind debts of about $280 million owed to employee pensions. The unpaid pension debts mean that some retirees will get smaller checks. Much of the tab will be picked up by the government’s pension insurer, a federal agency facing its own budget shortfalls. Founded by two onetime colleagues at Lehman Brothers, Marc Leder and Rodger R. Krouse, Sun Capital manages billions in private-equity investments, buying and selling companies for profit. The public face of the firm is Leder, a co-owner of the Philadelphia 76ers basketball team and the New Jersey Devils hockey team.