As pressure mounts on the government to order a recall of Firestone 500 steel-belted radial tires, the National Highway Traffic Safety Administration is negotiating with the Firestone Tire & Rubber Co. over the scope of a voluntary recall.

The major stumbling blocks, according to sources close to the talks, is Firestone's insistance that any voluntary recall include only tires manufactured before May 1, 1976. The company manufactured the tire for another two years after that date, but insists that technological changes in tires produced after the May 1 late make the tires much safer than earlier versions.

Consumer advocate Ralph Nader and House Commerce Subcommittee Chairman John Moss (D-Calif.) each sent letters to NHTSA administrator Joan Claybrook yesterday urging her to resolve the case, which has dragged on for three months since NHTSA made an initial determination that the 500 had a "safety-related defect" that led to thousands of tire failures.

Despite the fact that NHTSA says it has a case for a mandatory recall - one in which the government orders the company to replace all 500's still on the road at its own expense - it has attempted to negotiate a settlement with the tiremaker under which it recalls the tires on its own.

The reason for the negotiations, sources say, is the opinion by government regulators that Firestone would take such a recall order through a prolonged court fight, arguing that the government cannot pinpoint any specific defect.

But despite the fact that the government has repeatedly given Firestone attorneys deadlines to respond to "final offers," the company has returned time after time with counter-offers NHTSA felt worthy of consideration.

The company is trying to establish that some combination of a dozen or so technological changes in the 500 during its production life (1972-1978), resulted in tires with far fewer consumer problems.

The Moss subcommittee has said that one out of every six Firestone 500's ever made was returned by its owner to the company for replacement. In addition, the Moss subcommittee reported, one out of every 11 suffered tread separation, a condition that leads to blowouts and other problems.

But NHTSA has had considerable difficulty attempting to validate Firestone's claims that tires manufactured in later years were better than earlier ones, thus justifying a limited recall. NHTSA has already been involved in a prolongued court fight with Firestone to force the company to hand over an estimated 30 million documents relating to consumer complaint information.

NHTSA officials have said privately that their own investigations have given some support to Firestone's argument for a limited recall, but they have experienced considerable difficulty in pinpointing the date when the tires were improved enough to justify not recalling them.

The Center For Auto Safety, a consumer group active in the Firestone case from the beginning, added new controversy to the talks yesterday when it presented NHTSA with a case study of a Firestone tire failure in Alabama last August. In that case, Center director Clarence Ditlow pointed out, the failure involved a seemingly unprompted blowout of a Firestone 500 which was made in December, 1976.

Another problem has been one of communications. Firestone hired the Clark Clifford law firm specifically to negotiate with NHTSA on this matter. NHTSA officials point out that the law firm has been cooperative, but appears to have problems reaching the right Firestone officials. They said the firm has had to learn about the tire industry while negotiations are going on.

Critics argue that Firestone may be keeping the Clifford firm itself at a distance in an effort to stall negotiations.

In Nader's letter to Claybrook, a former asscoiate, he decries the "unconscionable delays which your agency has permitted regarding the long overdue recall of these tires. Accidents, deaths and injuries continue to cover the highways of this country because of the dangerous defects of these tires."

"NHTSA knows full well," Nader wrote, "that every day of delay saves Firestone at least $250,000. That many dollars worth of tires are not recallable because that many tires wear our or blow out on the average each day." He told Claybrook she was being "manipulated," by Firestone.

Moss' letter criticized Claybrook for both the delay and the secret nature of the negotiations, "which in turn has given rise to speculative and erroneous news stories."

Claybrook will only say that a negotiated settlement is close at hand.