General Motors Corp. yesterday admitted that it established a special task force to study the rear-braking characteristics of its controversial, 1980 model X-body cars in 1979 before the cars went on sale.

But GM denied that the panel was created to investigate alleged X-body braking defects or that the company lied to try to conceal its knowledge of such defects.

In papers filed yesterday in U.S. District Court for the District of Columbia, GM also denied government allegations that it knew or should have known that the 1980 X-car had defective brakes and that it secretly made safety-related changes in the X-car rear braking system after the car went into production in January 1979.

GM was responding to a $4 million Department of Justice lawsuit filed Aug. 3 on behalf of the National Highway Traffic Safety Administration. The six-count suit accuses GM of knowingly producing and selling 1980 X-cars that had faulty braking systems, of lying to the government at least 27 times about the problem and the company's handling of the alleged defect and of taking inadequate steps to correct the problem.

The government's petition asks the court to order GM to recall all of what NHTSA says are 1.1 million cars suspected of having the defect, manifested by rear brakes that sometimes lock in moderate-to-hard stops, causing the driver to lose control.

GM's denial yesterday of two of those charges--that it lied and that it had made secret safety corrections--was accompanied by a request that the court give the company another week to seek dismissal of the suit's first four counts, all of which involve safety issues.

NHTSA officials said yesterday that they had not seen GM's latest filing in the case and therefore could offer no reply.

GM also took issue with a NHTSA allegation that the company violated federal law by failing to include the NHTSA "hotline telephone number" in its recall notices in August 1981 for 47,000 1980 X-cars suspected of having defective brakes. The company acknowledged it did not include the number, but insisted it did not violate the law because the law was never properly implemented, GM said.

"NHTSA failed to promulgate the alleged Auto Safety Hotline regulation, in compliance with the requirements of the Administrative Procedure Act . . . and its own rules and regulations. Accordingly, the regulation is invalid," GM said in its filing yesterday.