The National Highway Traffic Safety Administration withheld vital information on braking defects in General Motors Corp. 1980-model X cars, even though the agency knew as early as July 1981 that the defects existed, a House subcommittee chairman charged yesterday.

An estimated 240,000 of the affected cars, most with manual transmissions, are suspected of having defective brakes that can lock and cause loss of driver control, according to latest NHTSA reports. The defects allegedly have contributed to 13 fatal accidents within the past three years, NHTSA said.

"You knew in July 1981 that there was a double problem" with the brakes, an angry Rep. Timothy E. Wirth (D-Colo.) told NHTSA Administrator Raymond A. Peck yesterday.

"You had the information . . . The evidence was very clear, but you didn't do anything about it. You had all of the data, but you sat on it," Wirth said. "You all were delinquent in fulfilling your responsibilities."

"That's unmitigated rubbish," Peck shot back. The NHTSA administrator, appearing before Wirth's consumer protection subcommittee, accused the chairman of misusing "partial facts" to buttress his charges.

NHTSA's handling of results from government tests on the affected GM X cars was at the center of the Peck-Wirth dispute. The tests were performed July 8 and 9, 1981, at a NHTSA facility in East Liberty, Ohio.

The 1981 test records indicate that the braking problems involve a proportioning valve, which regulates pressure on brakes, and the composition of rear-brake linings, Wirth said.

He said the records, excerpts of which were presented at yesterday's hearing, also show that "rear-brake lining composition contributes more to rear-brake lockup than proportioning valve" malfunctions.

But despite those findings, Wirth charged, NHTSA allowed GM to "whisper in its ear" and quietly recall some 47,000 X cars on Aug. 5, 1981, for the purpose of refitting the cars with smaller proportioning valves (the larger valves were suspected of allowing too much braking pressure).

The chairman produced another NHTSA document that, he said, proves that NHTSA knew before it conducted its first round of tests that changing proportioning valves "would have an extremely limited effect on brake lockup."

The document was a letter, dated July 8, 1981, written by former NHTSA defects investigations director George P. Anikis and addressed to Ralph C. Morrison, director of product investigations on GM's engineering staff.

Anikis said in the letter that GM's own findings on the affected X cars "indicated that these vehicles are equipped with rear-brake linings that are more aggressive than the linings used on 1980 X cars equipped with automatic transmissions." Cars with the "aggressive brake linings" and the heavier rear-brake proportioning valves "contain an engineering defect which has safety-related implications," Anikis said in the letter.

"In view of the seriousness of this situation and the continuing receipt of complaints by this office, I urge GM to review this matter," Anikis wrote.

Anikis said yesterday at the hearing that he "wanted to go after" GM because of the brake defects. But he said he was opposed by other NHTSA officials. "There were near fist-fights in the office" over what to do about the affected cars, Anikis said.

Nothing happened, at least publicly, until Jan. 14, 1983, when NHTSA issued an "initial determination" of defects in the cars.

Peck said NHTSA acted properly in not giving public notification of its July 1981 findings, and in not demanding a recall based on those findings. The evidence, at that time, was too skimpy, Peck said. GM, in possible legal retaliation, "would have hung this agency out to dry," Peck said.