''They are not quite up against the wall, but they are getting closer,'' said one securities analyst, who asked not to be identified. Another analyst, Charles Mancuso of Moody's Investors Service, said that to determine how much time to give the company to cover the missed debt payments, creditors must look closely at Greyhound's ability to increase revenues when the bus line's busiest season for passenger loads begins in June. Moody's Lowers Bond Rating Moody's lowered its rating of Greyhound's bonds today to Caa, a classification warning of possible default or of missed payments on principal and interest. The company is paying $780,000 a month in interest to its banks, with revenues from passenger fares on the rise, Mr. Seaquist said. A $25 million credit line with Security Pacific Bank in Los Angeles was drawn down several weeks ago. ''We've reached the limits of our borrowing capacity,'' he added.