The restaurant fire was small, a routine run that day in 1979 for the Lubbock, Tex., fire department. But suddenly it wasn't routine: three firemen went to check a smoky room and never came out. Their respirators were leaky, and they died of carbon monoxide poisoning.

At the Asarco plant in East Helena, Mont., a steelworker spoke up at a 1980 union meeting. When he cleaned the plant's respirator masks every day, he said, he noticed fine dust in the breathing tubes, well inside the filters that were supposed to stop the dust. The respirators were leaking.

Labor unions have begun collecting such incidents, trying to document their contention that too many "personal protective breathing devices" are leaking or don't live up to their billing. Textile workers, steelworkers, miners and asbestos workers say the failures prove that millions of lives are on the line in the Reagan administration's rewrite of rules on worker exposure standards.

Respirators are at the heart of the cost-benefit issue in regulating industry. The administration wants to make it cheaper for industry to meet exposure standards for lead, cotton dust, benzene, asbestos and other dangerous impurities, which traditionally require massive work place cleanups under rules of the Occupational Health and Safety Administration.

The administration first tried to lower costs by redoing exposure standards for lead and cotton dust through cost-benefit analysis. That failed when the Supreme Court ruled last June that pollution limits must consider only health effects, not costs. But the cotton dust decision seemed to leave the door open for cost-benefit analysis on the methods by which industry meets the standard, if not on the standard itself.

That makes respirators attractive. The American Textile Manufacturers Institute says that installing ducts, giant vacuum machines and other devices to clean textile plants of cotton dust could cost $2 billion nation-wide, or about $9,000 per worker.

The fanciest respirator, one with self-contained compressed oxygen tanks, a blower motor and a full face mask, like firefighters use, costs about $1,000, and textile workers would not need one that sophisticated.

Apparently responding to such numbers, OSHA changed the lead standard last month to allow the use of respirators for workers exposed less than 30 days a year to lead levels above the limits. Similar changes for other pollutants are reportedly in the works. The United Steelworkers of America plans to challenge the lead change in court.

The problem, according to the labor unions, is that respirators don't always work right. They may not fit tightly, especially over sweat, grease, beards, glasses, small faces or big noses. They impede conversation and workers often have to take them off to talk, to sneeze or cough.

Some make it harder to breathe, endangering the health of asthmatics or people with heart trouble. Other respirators are heavy, awkward or just plain uncomfortable. If they aren't scrupulously cleaned, maintained and their filters checked regularly, they may not function at all.

"I've never met anyone who could wear a respirator tightly strapped to their faces over an eight-hour shift," said Mike Wright of the steelworkers' union safety and health division. The only place for respirators, he said, is where engineering controls are not feasible, as in firefighting, demolition or paint spraying; for emergency uses, or in nonroutine operations like entering ventilators.

Even where respirators are the only recourse, as in demolition, workers may not be given the right type, said Roy Steinfurth, chief of the safety and health program of the Asbestos Workers International Union.

Protective, nonmedical respirators range from the small throwaway white gauzelike patches that cover nose and mouth and cost less than $1, designed to protect against ordinary dust, to the firefighters' specials designed to handle just about any kind of gas, smoke or mist. In between are half-face or full-face masks with chemical cartridges that filter outside air, and masks with hoods and hoses running to pumps 50 feet away.

In 1977, business spent $127.5 million on respirators, according to Department of Commerce figures.

OSHA rules specify what kind of masks go with what level of pollutants in the air, but the levels vary on a demolition site. Test results take a week or more to obtain and contractors sometimes want to minimize costs. "Workers think they're protected, but they sometimes aren't," said Steinfurth.

While few respirator failures have results as spectacular as the Lubbock deaths, leaks can mean death years later from cancer, said Vern McDougall, director of technical assistance and labor education at the Workers' Institute for Safety and Health, an independent arm of the AFL-CIO.

A Workers' Institute study issued in May said a third of all health citations from the National Institute of Occupational Safety and Health between 1972 and 1980 involved respirators. NIOSH checks of foundries in 1979 showed a third of them using respirators uncertified against metal fumes, and several other industries were found to be getting less protection from their respirators than the manufacturers promised.

The safety-gear industry defends its products, charging that most problems arise from improper maintenance. "In many instances, companies buy the respirators and expect them to give the ultimate in protection with absolutely no care," said Louis Rodenhouse of Scott Aviation Co., chairman of the respirator group of the Industrial Safety Equipment Association.

But NIOSH has taken stop-sale or recall actions for structural defect reasons against 30 respirator models since 1979, according to its reports. Unionists worry that budget cuts and the Reagan administration shift in emphasis may frost that effort.

OSHA is about to review its respirator standards to include language on fit testing methods, training and screening of wearers, protection ratings and productivity effects. NIOSH is rewriting its certification program under some prodding from the unions. "In the past no one monitored them, no one pushed the manufacturers, nobody gave a damn," said Smith. "After Lubbock, we started raising hell."