The National Highway Traffic Safety Administration's four-year investigation of dangerous braking defects in General Motors Corp.'s 1980 X-body cars was marred by needless delays and sloppy procedures, the General Accounting Office reported yesterday.

"GAO found serious problems in the safety administration's handling" of the defect investigation, a GAO report said.

"Although the safety administration had written guidelines for conducting the engineering analysis and the formal investigation phases, GAO noted several instances where problems developed" in the investigation "because the guidelines either made no provision or were not clear with respect to taking certain actions," said the report, which was discussed at the hearing by J. Dexter Peach, director of GAO's community and economic development division.

Rep. Timothy E. Wirth (D-Colo.), who asked for the GAO probe, said the NHTSA's failure to act quickly in the brakes case may have contributed to 15 traffic fatalities since the 1980-model X cars were introduced.

"While NHTSA was going slow on the X-car probe, 15 people have been killed, 71 Americans have been injured and nearly 2,000 more have complained to the government," Wirth said. "This is a sorry state of affairs, an inexcusable track record for an agency whose primary obligation under the law is to remove dangerous vehicles from our roads."

The defect in the 1980 Chevrolet Citation, Pontiac Phoenix, Oldmobile Omega and Buick Skylark sometimes causes the rear brakes to lock in moderate-to-hard stops, causing loss of control. Allegations of fatalities related to the defect have been made in individual complaints to NHTSA. Agency officials say there has been no official determination that the deaths resulted from the defective brakes.

The GAO report on NHTSA enraged Democratic members of the House subcommittee that oversees the safety agency. No Republican subcommittee members attended the hearing.

The alleged behavior of NHTSA and GM in the matter, in which GM "lied and covered up," constitute a "classic stonewall," Rep. Edward J. Markey (D-Mass.) said.

GM officials "should be asked to turn in their MBAs and start business ethics classes all over again; their cavalier behavior endangered too many lives to go unpunished," Markey said.

GM officials declined to appear before the subcommittee, which held its hearing two days after NHTSA filed a $4 million suit against the nation's largest automaker.