A chronology of the X-Car braking controversy, from documents filed with National Highway Traffic Safety Administration:

January, 1979: General Motors Corp. starts production of X-bodies.

March 18, 1979: GM approves production change from "aggressive" brake lining, pads with higher friction per square inch, to "nonaggressive" brake lining, pads with lower friction per square inch. The change is approved for all 1980 X-cars with automatic transmissions. Neither NHTSA nor the public knows of the change.

April, 1979: X-bodies go to showrooms; sales begin.

May, 1979: The National Highway Traffic Safety Administration's Office of Vehicle Safety Compliance (OVSC) begins two tests of X-body brakes. The cars pass the first test--the required emergency braking check--but the "passes" mostly are marginal.

Federal regulations require that brakes be capable of stopping a lightly loaded car traveling 60 miles per hour within 194 feet from the initial braking point.

The Buick Skylark and Oldsmobile Omega stop within 185 feet. The Pontiac Phoenix stops within 193 feet. The Citation stops within 175 feet.

The second group of tests are ordered because of "media criticism and consumer complaints of X-car stability during braking," according to a June 26, 1981, internal NHTSA memo. The cars "generally" stayed within their 12-foot-wide lanes, the NHTSA memo said. "One exception to this involved one of the manual transmission cars . . . When a more severe stop was attempted . . . both rear wheels locked, sending the vehicle into a skid the test driver was unable to correct," the memo said. GM officials later used the passing grades to argue that their X-car brakes were problem-free and government-approved.

But there was no publicity of the following footnote on the test results: "It is important to keep in mind that the above tests were conducted under ideal conditions, on a high coefficient of friction very dry surface that is straight and within 1 percent flat. The test drivers were skilled, and there was very little brake lining wear" on the test cars, the NHTSA memo said.

May 1, 1979: GM begins switching to non-aggressive linings in production and service stock to "improve rear-brake heat sensitivity," GM says in subsequent correspondence with NHTSA. NHTSA Receives First Complaint

July 9, 1979: NHTSA receives first owner complaint of X-Car rear-brake lockup causing loss of driver control.

Aug. 27-29, 1979: GM begins full-scale change of X-car brake proportioning valves (two on each car) in production and service replacement stock.

The valves, along with the "aggressive" brake linings, are the keys to the safety controversy. Most stopping pressure goes to front brakes in front-wheel-drive cars because most of the weight is up front. Less force is applied to the rear brakes where the load is lighter. If too much braking pressure is applied to the lightly loaded rear brakes, the brakes can lock up prematurely, forcing the car to spin out of control.

GM's initial X-car braking arrangement allowed the rear brakes to take 41 percent of the braking pressure over 350 pounds of pressure per square inch (psi). The steel, 41 percent proportioning valves were gradually replaced with aluminum valves that reduced rear braking pressure to 27 percent in excess of 350 psi.

Nov. 26, 1979: NHTSA's Office of Defects Investigation (ODI), responding to 10 consumer complaints, opens an engineering analysis of the alleged brake lockup problem. X-car braking is also getting criticized in the auto trade press by this time. An engineering analysis is the first step toward product recall.

December, 1979: GM's Buick Division sends "service information bulletin" to Buick/Opel dealers, announcing a change to aluminum proportioning valves 27 percent instead of the steel valves 41 percent but saying nothing about braking difficulties. Nor does it explain why the proportioning valves were being changed.

Dec. 10, 1979: NHTSA conducts "subjective road tests"--tests without instruments--to evaluate vehicle directional stability in various braking patterns at various speeds. NHTSA's safety compliance office says the test car, a 1980 Chevrolet Citation, swerved to the left, and sometimes to the right at high-speed stops. GM: Brake Valves Not a Problem

May 27, 1980: Lynn L. Bradford, acting director of NHTSA's Office of Defects Enforcement (in ODI), sends letter to GM's Ralph C. Morrison, director of product investigations, telling him that NHTSA now has "23 reports of rear-brake lockup and skidding in 1980 X-body vehicles." Bradford's letter is a request for information on the alleged defect to determine if a safety problem exists.

July 21, 1980: Morrison responds: "General Motors has not conducted any recall campaigns, issued any service bulletins, made any warranty changes or issued other instructions to dealers involving rear brake lockup in the subject vehicles. The only production change that may be responsive to this question involved a variation in the proportioning valve function from 0-350 psi X 41% to 0-350 X 27%."

Jan. 2, 1981: James P. Talentino, chief of the engineering analysis division in ODI, signs letter to GM's Morrison asking for updated information on rear-brake lockup problem.

March 11, 1981: Morrison responds, saying: The proportioning valve change was made not for safety reasons, but because GM found that X-car owners typically carry lighter loads in the trunk and fewer rear-seat passengers than GM originally had assumed.

GM had done no testing "relative to the subject brake problem"; the "alleged brake problem . . . does not constitute an unreasonable risk to motor vehicle safety." The X-cars passed all of NHTSA's standard brake systems compliance tests. X-cars are "no more prone" to rear-brake lockup than are other front-wheel-drive cars. (GM offered no documentation on this point.)

April 9, 1981: According to the General Accounting Office investigation of NHTSA's handling of X-car probe, ODI engineer Larry Cook, chief engineer in charge of the X-car defects probe, "first became aware that more agrressive brake linings had been used on manual transmission 1980 X-body cars which could account for most of the complaints about rear brake lockup."

June 2, 1981: NHTSA and GM have a total of 212 complaints about rear-brake lockup in X-cars. Accidents, 14 of which involved injuries, are alleged in 58 of the complaints. NHTSA has one report of an accident-related death stemming from rear-brake lockup. Agency Officials Visit GM

June 3-4, 1981: NHTSA officials visit GM engineering staff in Warren, Mich. According to a memo written by Gary R. Woodford, safety defects engineer at ODI, "Mr. George P. Anikis NHTSA defects office chief and I both expressed concern over the continuing number of owner complaints received by the ODI on this subject, as well as the number and severity of alleged accidents."

"Mr. Morrison admitted that there is a problem; however, he questioned whether it was safety-related. He also indicated that there had been a parking brake system redesign on the X-car to preclude the need for aggressive rear-brake lining," Woodford wrote.

"Finally, Mr. Anikis urged the GM personnel to review this matter and take appropriate steps to resolve the problem by recalling the vehicles. Mr. Dale Johnson stated that GM would objectively review the new complaints and take action as appropriate," Woodford wrote.

July 1, 1981: NHTSA's defects review panel decides to upgrade the X-car inquiry to "a final defect investigation . . . involving rear brake lockup in all 1980 X-cars," according to a memo written by Talentino on July 13, 1981.

July 6, 1981: Anikis notifies Morrison that NHTSA believes "the rear brake system of the 1980 X-body vehicles (utilizing 41 percent valves and aggressive brake linings) contains an engineering defect which has safety related implications."

"In view of the seriousness of this situation and the continuing receipt of complaints by this office, I urge GM to review this matter. Your response to this letter, stating what corrective action GM plans to take, is requested within five working days," the Anikis letter says.

The letter is a clear invitation to GM to initiate a voluntary recall, according to NHTSA and other auto safety sources.

July 8, 1981: Morrison responds to Anikis' letter of July 6, 1981, saying that GM would initiate a voluntary recall of 1980 X-cars with 41 percent valves and aggressive rear-brake linings.

The letter says in part: "General Motors does not agree with your assertion that the subject vehicles contain 'an engineering defect with safety-related implications.' However, to preclude the possibility of prolonged and costly legal procedures which could ensue based on your apparently adamant position in this matter, General Motors will initiate a recall modification relative to the involved vehicles."

The recall affects 47,371 cars. The fix is to change proportioning valves on the cars that missed the production valve change, but not the remaining "aggressive" brake linings.

Nov. 4-19, 1981: NHTSA tests 1980 Chevrolet Citation to determine the effects of aggressive linings on X-car braking.

Nov. 13, 1981: Based on a preliminary analysis, NHTSA's case engineers conclude that "the greatest contributor to the 1980 X-body cars' rear-brake lockup problem was the aggressive linings," according to a GAO study of the case. The implication is that the August, 1981, X-car recall was ineffective and that the affected cars would have to be recalled, again. The finding also meant that, possibly, all 1980 X-cars might have to be brought back for fixes.

Jan. 11, 1981: ODI receives the first consumer complaint that the replacement of the proportioning valve did not correct the X-car's braking problem.

June 21, 1982: NHTSA receives the final report on the results of November, 1981, tests of the Chevrolet Citation. The report suppports the engineers' earlier conclusion that aggressive brake linings are a major contributor to lockup and spinout.

Oct. 15, 1982: The Center for Auto Safety asks NHTSA to extend the X-cars brake investigation "to include all GM X-cars regardless of the date of manufacture." The letter marks the beginning of a major public campaign for an X-car fix.

Nov. 1, 1982: Anikis is transferred from his job as ODI director to the Office of Traffic Safety Programs. George L. Parker takes over as acting ODI director. NHTSA Orders Extended Probe

Dec. 8, 1982: Bradford, NHTSA associate administrator for enforcement, grants the auto safety center petition for an extended X-car investigation.

Dec. 17, 1982: Parker writes GM saying that the August, 1981, GM recall and the GM corrective action have "not completely solved the rear-brake lockup problem."

Jan. 7-13, 1983: NHTSA's November, 1981, X-car test results and July, 1981, X-car test results are placed in the agency's public file, largely at the urging of Rep. Timothy E. Wirth (D-Colo.), chairman of a House Energy and Commerce subcommittee. Wirth's prodding of NHTSA increases pressure on the agency to take stronger action in the matter.

Jan 14., 1983: NHTSA's Bradford writes GM President James McDonald, telling him that NHTSA has made an initial determination of defect, that "all 1980 X-body vehicles equipped with either manual or automatic transmissions which were equipped with asbestos-lined rear brake shoes are subject to failures in performance which can result in accidents, injuries, death or property damage."

NHTSA's investigation indicates that the affected cars include "all manual transmission X-body vehicles and all automatic transmission X-body vehicles built before May 1, 1979," Bradford said.

The NHTSA official added: "The evidence also indicates" that the remedy--replacement of proportioning valves--"has not corrected the condition which may lead to such failures in performance."

The letter is another invitation, albeit sterner than the one written by Anikis in 1981, for GM to initiate a voluntary recall.

NHTSA on the same day issued a news release announcing its "initial determination of defect" in 1980 X-car brakes. GM Agrees to Second Recall

Feb. 9, 1983: GM Attorney William L. Weber Jr. writes NHTSA Chief Counsel Frank Berndt, advising Berndt that "General Motors has decided to recall all 1980 'X' cars equipped with manual transmissions and certain early production 1980 'X' cars equipped with automatic transmissions." Weber's letter to Berndt is in response to Bradford's Jan. 4 letter.

"The conditions described in Mr. Bradford's letter will be repaired at no charge to owners," Weber wrote.

This second brake-related recall affects 239,799 X-cars, including those recalled in August, 1981, for the proportioning-valve fix.

Feb. 25, 1983: Berndt writes GM's Weber, seeking more information on the recall and the nature of the second fix.

March 4, 1983: Berndt sends hand-delivered letter and "Special Order and Document Production Request" to Morrison, via GM's Washington office. This step is taken because NHTSA investigators found that GM provided "incomplete" answers to the agency's Dec. 17, 1982, the Parker letter information request, Berndt said.

"In other instances, requested information was not furnished at all," Berndt said in the March 4 letter. He added: "We stongly urge you to fully and promptly comply with the terms and spirit of this letter and accompanying Special Order."

The letter marks NHTSA's strongest action in the X-car braking case. Agency sources say the letter also marked the beginning of NHTSA's preparation to file a federal suit against GM in the X-car matter.

March 25, 1983: GM's Morrison sends sworn response to the Special Order. Among other things, Morrison said that, beginning in 1979, GM made three changes in the 1980 X-car braking system "which may have an effect on front to rear brake balance." At least 12 changes were made in the 1980 X-car parking brake system, Morrison said.

March 30, 1983: NHTSA issues news release announcing the second recall of 1980 X-cars in the brakes case.

Aug. 3, 1983: NHTSA, through the Department of Justice, files a $4 million suit against GM, alleging that the company began producing and selling 1980 X-cars even though it knew the cars had defective brakes.

"By December 1978, General Motors was aware, as a result of testing conducted on prototype models of the X-car before the beginning of production, that the X-car, as designed at that time, had a tendency to experience premature wheel lockup under a variety of conditions," the suit said.

The suit asked that GM recall all 1.1 million of its 1980 X-bodies. The petition, the first filed by NHTSA seeking civil damages against an auto maker, also accused GM of furnishing "false and misleading responses to NHTSA's information requests" in at least 27 instances.

That same day, Wirth issues a statement accusing NHTSA of covering up its X-car investigation. Wirth implies that NHTSA's suit against GM was filed to reduce the impact of a forthcoming General Accounting Office report on NHTSA's handling of the case.

Also on Aug. 3, 1983, GM's Weber issues a statement, accusing the government of broadsiding the company with its suit.

"We are surprised by the unexpected filing of the Justice Department suit. It is especially unwarranted in view of the fact that GM has cooperated extensively with NHTSA to develop the facts which will show clearly that no further recall or corrective action is appropriate. GM has voluntarily recalled about 240,000 vehicles. We believed such action was appropriate to allay customer concern. Also, we categorically deny the government's assertion of misrepresentation. Accordingly, we will vigorously defend against the lawsuit."

Aug. 5, 1983: Wirth's subcommittee holds hearing on the GAO's report, which says that NHTSA's X-car probe had "serious problems," including "long periods of time with no activity." Key parts of the investigation " [were] not conducted in accordance with applicable guidelines." Only Democratic subcommittee members attended the hearing, at which NHTSA and GM were accused of disserving and deceiving the public.