General Motors Corp. yesterday agreed to recall 3.5 million of its most popular sport-utility vehicles, vans and pickup trucks that may have faulty anti-lock brakes that fail to stop quickly under certain road conditions.

The company agreed to recall the vehicles after a five-year investigation by federal safety regulators, who received almost 11,000 complaints about the performance of the brakes, as well as reports of 2,111 crashes and 293 injuries for vehicles manufactured during the 1991 to 1996 model years.

For years, many problems with anti-lock brakes have been dismissed as improper use by drivers who are used to pumping their brakes to slow and avoid skidding. Anti-lock brake systems require drivers to press down firmly on the brakes and keep the pedal down until the vehicle stops. Anti-lock brakes, when used properly, avoid allowing the car to go into a dangerous skid or tailspin.

The problem resulted in some cars taking up to 50 feet longer to stop, according to the National Highway Traffic Safety Administration, which conducted the investigation.

The recall is not the largest by NHTSA, but government officials consider it significant because the investigation by the safety agency was one of its most complex, involving the interactions between computer hardware, software and human use of the braking technology, which is designed to keep brakes from "locking up" and causing the vehicle to skid when tires hit patches of ice or slick pavement or when the brakes are needed for sudden stops.

GM said the investigation was difficult because many consumer complaints were about "brake feel." Bob Lange, GM's director of safety engineering, said it was tough for GM and NHTSA to find common threads in their problems, such as broken parts or equipment that would leave a trail of evidence.

In many of the tests, NHTSA investigators said the stopping time on the brakes was extended an additional 40 to 50 feet, meaning the vehicle might rear-end the car in front of it, hit the sidewalk or slide into an intersection.

"The software didn't send the wrong signal every time, but it happened enough that we were concerned about it," said a top NHTSA official.

GM said that there was a safety defect in 1.1 million four-wheel-drive GMC Sonomas, GMC Jimmys, Chevrolet Blazers, GMC S-10 pickups, and Cyclone and Typhoon trucks manufactured for model years 1991 to 1996. The automaker said owners should take the vehicles to GM dealers, who will change a sensor switch that sends out faulty readings.

Basically, when the vehicle was operating in two-wheel drive, the system believed the truck was in four-wheel drive, a condition that could extend stopping distances from 10 percent to 30 percent, said GM's Lange.

In another 2.4 million two-wheel-drive trucks covering model years 1993 to 1996, and certain Chevy Astro, GMC Safari and G vans in model years 1992 to 1996, GM said the anti-lock brake system could, but rarely did, malfunction.

Among those vehicles, GM said adjustments would be made in the computer system that was misreading what kind of pavement the vehicles were on. The trucks with that problem include the Blazer, Jimmy, S-10 and Sonoma.

Lange had no estimates on the cost of the repair work. In some cases, repairs can be made immediately at dealers, but in other cases the brake manufacturers are still not ready to make the necessary computer-related changes.

NHTSA and GM said a second investigation into anti-lock brakes' performance in GM's Suburban vehicles produced in the 1992 to 1994 model years will remain open.

The agency has received 2,400 reports of different kinds of brake problems with Suburbans, as well as information on 782 crashes and 68 injuries believed to be related to anti-lock brakes' performance.

The 1.1 million four-wheel-drive vehicles are subject to the government recall order. Owners of the other 2.4 million vehicles will receive a special service recall notice from GM asking them to bring their vehicles in for free repairs. NHTSA officials said owners should treat the service notice the same as a government recall.