Nearly 1.5 million Volkswagen and Audi cars on U.S. roads may have serious braking or electrical system defects, the National Highway Traffic Safety Administration warned yesterday.

The cars, built between 1975 and 1982, were manufactured in Germany and sold in the United States through VW's domestic subsidiary, Volkswagen of America.

None of the cars suspected of having the defects was produced at VW's Pennsylvania plant, the government said.

NHTSA's announcement, stemming from two separate auto safety investigations, comes in the midst of a multimillion dollar ad campaign touting VW's heritage of German engineering. The campaign was conceived by VW officials who felt that the company's slumping U.S. sales partly are influenced by consumer suspicion that VWs made in America are inferior to their German counterparts.

VW officials could not be reached for comment last night. But NHTSA said the company has denied the existence of safety defects in both cases where the government has made an initial determination of defect.

The first finding covers 650,000 VW Rabbits an Sciroccos made in model years 1975 through 1980. These cars are suspected of having hydraulic brake lines that rust and rupture.

The second finding affects about 930,000 VW and Audi cars made in model years 1975 through 1982. These cars may be equipped with fuel injected gasoline engines that can experience electrical system problems, NHTSA said.

In the case of the troubled brakes, "Rupture . . . may result in a sudden increase in the distance required to stop the vehicle when the brakes are applied. There have also been reports of complete loss of braking when rupture occurs," NHTSA said in its announcement.

Initial determinations of defect by the agency in the past have frequently lead to voluntary recalls by manufacturers. The government can seek a final determination of defect--an involuntary recall--if it feels the defect has not been remedied.

The agency said it has received 96 owner complaints, including reports of eight property damage and three personal injury accidents allegedly caused by hydraulic brake line failure in the affected cars.

The problem seems to be a design flaw, in which the hydraulic lines to the rear brakes are routed along the floor inside the passenger compartment of the affected cars, NHTSA said. Moisture can collect between the car floor and carpet, cling to the lines and cause then to rust.

NHTSA said its investigation of possible electrical system defects "is the result of more than 3,000 consumer complaints, including reports of stalling, inability to start the engine, detection of smoke, fumes or electrical fires cause by overheated components."