Sources now say that all of General Motors Corp.'s 1980-model X cars with manual transmissions--some 200,000--may have defective brakes that can lock in emergency stops.

The problem was confirmed on Wednesday by the National Highway Traffic Safety Administration, but the extent of the possible recall was not disclosed.

General Motors failed to correct the alleged defect in a recall of some of the cars--which have manual transmissions--and in a production change made after the recall, the sources said.

The NHTSA is considering ordering GM to recall the cars to make changes in the brake systems, which could cost the automaker as much as $30 million. GM already has recalled 47,000 of the popular front-wheel-drive X cars made in the 1980 model year, but the government says GM's repairs apparently didn't solve the problem, which also appears to affect all 1980 models with manual transmissions, not just those recalled.

GM has said it does not "believe X-car braking systems present an unreasonable risk to safety," but is cooperating with the NHTSA investigation.

Locking rear brakes during panic stops can send the car into a skid or spin. At least one death and several injuries have been attributed to accidents allegedly caused by the problem.

The nearly 1 million X cars with automatic transmissions made in the 1980 model year are not nearly as vulnerable to brake lock-up as those with manual transmissions. Sources say complaints to the government by owners of manual-transmission X cars far outweigh those from owners of the cars equipped with automatic transmissions. GM makes X cars under the Chevrolet Citation, Oldsmobile Omega, Pontiac Phoenix and Buick Skylark nameplates.

Sources say that General Motors waited until the end of the 1980 model year to make the needed change in the design of the cars. The change involved installing a lighter-weight rear-brake lining than had been used in the 1980 models. As a result, X-body cars made from the 1981 model year on apparently don't have the problem. But the 200,000 manual-transmission X cars made in the 1980 model run--including those recalled--still may be susceptible to the locking brakes.

A General Motors spokesman had little comment on the charge, although he did confirm that technical changes were made to the braking system between the 1980 and 1981 model years. He would not discuss those changes, saying they were an issue in the NHTSA's ongoing investigation of the X-car braking problem.

Sources say an NHTSA ruling on that investigation is imminent, possibly including an order that GM recall the cars to fix the linings.

The alleged problem with the manual-transmission X cars centers on the power of their rear brakes. Sources say the proportioner valve--which divides hydraulic force between front and rear brakes--and the rear brake linings on the cars were designed to provide much more stopping force than needed under ordinary conditions. As a result, when the brakes are jammed on, they can lock. X cars with automatic transmissions have different brake settings that minimize the problem.

GM started building X cars in April 1979. In July of that year, the NHTSA started investigating consumer complaints that the rear wheels of the cars locked, and in August 1979, GM voluntarily recalled 47,000 of the cars that had been equipped with manual transmissions.

In that recall, GM replaced the proportioner valve with one that delivered less power to the rear brakes. The same change was made in cars assembled after that point.But sources say GM did not change the cars' heavy-duty brake linings, and the lock-ups continued, even on the cars that had been recalled.