General Motors Corp.'s X-car trial, a sputtering legal affair that seems incapable of reaching top speed, resumed here yesterday after a three-month recess.

At issue is whether GM deliberately rolled out 1.1 million 1980-model X cars with potentially defective rear brakes that could lock up and cause loss of driver control.

GM says the cars are safe. The National Highway Traffic Safety Administration says in a suit filed Aug. 3, 1983, through the Department of Justice that the cars are dangerous and should be recalled immediately.

Also at issue is the government's charge that GM lied to cover up the alleged defects. GM says it told the truth and cooperated with NHTSA in good faith. But the government is seeking $4 million in damages from GM on the ground that the auto maker's alleged technical and ethical misdeeds have served to injure public safety.

The technical and ethical questions are being handled separately.

The case to resolve the technical matters through a recall began March 13. That is the case that resumed yesterday.

The legal inquiry into whether GM lied is to begin after the technical case ends. Both are civil, nonjury proceedings before U.S. District Judge Thomas P. Jackson.

Attorneys for both sides say the the first trial is moving slowly -- very slowly -- largely because of the need to present voluminous technical material to buttress their respective arguments.

To undertand that material, Jackson has to be turned into a brake expert -- a task made all the more difficult because Jackson must handle numerous other cases at the same time he is trying to familiarize himself with car-brake theory, function and design, the attorneys say.

As part of its educational contribution yesterday, GM put William H. Gillespie, a senior staff analysis engineer from the GM Technical Center in Warren, Mich., on the stand.

Gillespie is the third of 12 GM experts who are scheduled to testify in the technical trial. The gist of his remarks, charts and two video presentations yesterday was that GM's brake-testing procedures and instrumentation in the development of the front-wheel-drive X cars were more than adequate to produce a safe-braking car.

Meanwhile, NHTSA is investigating alleged rear-brake defects in 1979 Mercury Capri and Ford Mustang passenger cars, both rear-wheel-drive vehicles produced by Ford Motor Co.

An NHTSA official said yesterday that the agency had received 160 complaints of rear-brake lockup in the 1979 Capris and Mustangs as of April 10. The complaints included reports of 54 accidents and 15 injuries, the spokesman said.

A Ford spokesman yesterday said his company is "cooperating with NHTSA . . . and is confident that the agency will reach our conclusion that there are no defects" in the cited Ford cars.