THE MBTA is preparing to cut service levels to deal with a budget shortfall looming over next year, but the Massachusetts Taxpayer Foundation warned members of the Fiscal and Management Control Board on Monday that even bigger problems are just around the corner. Andy Bagley, a vice president at the Massachusetts Taxpayers Foundation, said MBTA capital funding sources will fall off a cliff in fiscal 2025, roughly four years from now. When that happens, he said, the T won’t have enough money to maintain and modernize the existing system and pursue other initiatives that already have wide support.