While much of official Washington was off on vacation, lawyers at the National Highway Traffic Safety Administration have been putting in long hours preparing for their upcoming--and probably uphill--legal battle against General Motors Corp.

The agency, through the Justice Department, has filed a $4 million lawsuit against the nation's largest automaker, charging the company with lying at least 27 times in an attempt to conceal a braking defect in its 1980 model X-cars.

The suit, filed on Aug. 3, also seeks to force the recall of 1.1 million cars--including the 1980 Buick Skylark, Chevrolet Citation, Oldsmobile Omega and Pontiac Phoenix--suspected of having the defect. The problem involves rear brakes that can lock in moderate-to-hard stops, endangering driver control.

GM has denied the charges, and so far has won a procedural skirmish in the fight. The U.S. District Court here granted the company a two-week extension of its original 20-day period to answer NHTSA's complaint. GM now is scheduled to respond by Sept. 7.

GM's motion for extension was filed Aug. 23, a day after government lawyer Lawrence Moloney, in a telephone conversation with GM attorney Theodore Souris, refused to agree to a delay. GM said it needed the extra time to determine "whether to file an answer or, if it be appropriate, a motion either for summary disposition or to stay this case pending completion of administrative proceedings before NHTSA."

The motion indicated a part of GM's overall defense strategy. The company will argue that it was blindsided by the NHTSA suit, that the government's action was taken without due regard to administrative procedure.

NHTSA and Justice Department lawyers declined comment. But Reagan administration sources familiar with the case say NHTSA does not have to seek an administrative remedy to get GM to recall all of the affected X-cars.

NHTSA's argument is that GM knowingly misled the government and the public in August, 1981, when the company recalled 47,371 of the affected vehicles. Subsequent investigations by the agency, according to the government's lawsuit, show that GM knew that the 1981 recall was inadequate, both in terms of the number of cars called back for repairs and in terms of the repair, which involved replacement of certain brake valves.

GM also knew that another recall of 240,000 X-cars in February, 1983, would not do the job of correcting braking problems, according to the suit. According to one source, "It is not a matter of going through administrative procedures to determine that a defect exists. It is a matter of showing that GM knew the problem existed, misrepresented facts, and failed to take adequate action to correct the problem."

GM attorneys met with Justice Department and NHTSA attorneys on Aug. 12 to ask that the government produce all documents upon which the misrepresentation charges were based. The government refused that request on Aug. 16, saying that it would not disclose any evidence, except through the formal process of discovery. On Aug. 19, the company formally petitioned Justice to begin discovery proceedings.

The company said it is insisting on a pretrial disclosure of evidence because "the charges made in this suit are so serious, impugning the reputation and the integrity of GM by alleging it made false or misleading statements to NHTSA during the course of a then and still pending administrative proceeding investigating alleged safety defects in GM's X-cars."