The National Highway Traffic Safety Administration yesterday issued a notice that 320,000 of General Motors' X-body front-wheel-drive cars made during the 1980 model year have defective brakes and may be recalled.

The notice covers 47,000 cars GM has already recalled once for the same problem.

NHTSA, which had been expected to issue the defect finding in the wake of hundreds of complaints about the cars, called a hearing for Feb. 14 to ask GM why it shouldn't be ordered to recall the cars.

General Motors, in a statement from Detroit, said, "We are continuing our evaluation of the cars in question, and as previously stated, are prepared to make corrections if necessary." It said it would respond to the NHTSA charges at the Feb. 14 hearing.

Cars involved are the Chevrolet Citation, Pontiac Phoenix, Oldsmobile Omega and Buick Skylark. All 220,000 cars made during the 1980 model year with manual transmissions are affected by the government announcement, as well as 100,000 equipped with automatic transmissions. GM made more than 1.1 million X cars in the 1980 model year.

The alleged defect apparently doesn't occur in later-model X cars, but NHTSA said it was continuing to investigate complaints made about brake lock-up in later models.

The government said the 1980 X cars' rear brakes have a tendency to lock under moderate-to-hard braking conditions. At least one death and more than a score of injuries have been caused by crashes allegedly linked to the problem.

According to NHTSA, the defect lies in the thickness of the brake-linings installed on the cars, and on the valves used to proportion hydraulic power sent to the brakes. GM allegedly used extra-thick brake linings in the rear brakes of all manual-transmission X cars built in the 1980 model year, and on some of the cars with automatic transmissions, giving the cars unusually strong braking power. Sources say complaints about wheel lock-up began almost immediately after the cars were introduced in April 1979.

The company apparently made changes in the cars' design as production continued, going to a thinner brake lining on automatic-transmission models and reducing the power of the proportioning valves on all models. In August 1981, GM recalled 47,000 early-1980-model X cars with manual transmissions and installed the newer proportioning valves.

In addition, NHTSA extended the defect notice to automatic-transmission X cars built before May 1979 because of a high number of complaints about lock-up.