General Motors Corp. and the National Highway Traffic Safety Administration yesterday announced a $30 million recall campaign to repair possible brake defects in 240,000 of GM's 1980 X-body cars.

Repair work could take from 2 1/2 hours to 5 1/2 hours per car, "the most extensive work per car ever involved in a recall," NHTSA Administrator Raymond A. Peck Jr. said yesterday.

GM said it would start sending recall letters to car owners April 4, but parts for the repairs will not be available until about April 25.

Peck said owners should take their cars to dealers as soon as possible after receiving the notices.

"This is a very serious safety related problem. There should be no question in anyone's mind that the owners should return these vehicles to the dealership for correction when they receive their recall notice," he said.

Rear brakes in the affected cars tend to lock up in moderate to hard braking, "which could cause the cars to spin out of control without warning," according to a NHTSA statement yesterday.

The defect has contributed to 13 fatalities in the last three years, according to NHTSA.

Affected cars include 1980 models of the Chevrolet Citation, Pontiac Phoenix, Oldsmobile Omega and Buick Skylark, most of which are equipped with manual transmissions.

Maximum repair work on affected vehicles would include:

Replacement of the rear brake shoes with shoes that have a different lining material;

Replacement of the parking brake cables on all problem cars with manual transmissions;

Inspection, lubrication and adjustment of parking brake cables, if adjustment is needed, on cars with automatic transmissions;

Replacement of other parking brake components, if needed;

Cleaning and inspection of rear brake drums, and repairing or replacing rear brake drums, if necessary;

Inspection of front brake system, and repairing or replacing any parts needed to provide uniform braking power to the entire brake system.

GM agreed to a recall of 47,000 of the 1980 X cars in August 1981. The company at that time replaced proportioning valves in an attempt to fix the braking problem.

But, according to numerous customer complaints to NHTSA, the initial repair was inadequate. Subsequent investigations by NHTSA and GM indicated that adjustments should have also been made in the rear brake linings of the cars.

The proposed strategy was to replace so-called "aggressive linings" with weaker linings that would provide adequate braking power without locking the brakes. But that approach led to possible problems with the parking brakes.

The expanded fix should correct all of the braking problems GM said.