When Rep. Barbara Boxer (D-Calif.) recently caught the Defense Department spending $7,400 for a coffee brewer for its C5A cargo plane, dismayed Air Force officials ordered an investigation.

They found that the Air Force, not the contractor, was at fault.

Their investigation showed that the Air Force had bought three brewers for $7,400 each last year. The contractor, Weber Aircraft, was following Air Force instructions to create a "hot brewing machine" that was, as Lockheed Corp. said in a statement, "the first of its kind and capability."

This was a machine that would keep working even after the plane had lost all cabin pressure. This was a coffee brewer that could keep brewing under the force of 40 g's of gravity -- enough to kill all the coffee drinkers on board.

This was a coffee brewer with more than 2,000 parts, custom-designed and custom-built to military specifications. For each unit, the contractor's charges included $2,856 for materials, $1,181 for 137 hours of assembly labor, $1,760 for utilities and other overhead, $718 for administrative costs and $548 for profit -- a "fair and reasonable price" for the Air Force.

"It's ludicrous to pay that much for the pot," said Air Force Gen. Bernard L. Weiss, director of contracting and manufacturing policy, in an interview yesterday.

Weiss said the coffee-brewer story shows much about problems that have plagued military procurement. But he said it also shows that Air Force officials are aware of those problems and unlikely to let similar mistakes slip past.

"I honestly don't believe we would let this happen to us today," he said. "I believe the system is so sensitive today that it would jump up and vomit."

Nobody noticed the coffeepot price until a young airman at Travis Air Force Base complained to outside reformers. Weiss attributed the lapse to what he called the "ordering mentality."

"When you're ordering, you're concerned with only one thing: when will the part arrive," he said. "We're getting rid of the ordering mentality, we're moving to the buying attitude."

That means, Weiss said, that the Air Force's 13,000 employes concerned with procurement now worry about price and quality as well as convenience. They are furnished with drawings of parts so they know that a "hexagonal alignment tool" is an allen wrench and a "hot beverage unit" is a coffee brewer.

Buyers are authorized to jawbone manufacturers to obtain a reasonable price, he said, and to recommend buying elsewhere or manufacturing in-house if the contractor does not respond.

In addition, the Air Force has begun to study the problem of "overspecification." That means, Weiss said, that officials will begin to raise questions when someone says a coffee brewer must keep working after a crash.

Weiss said that the reforms have begun to work and that almost none of the parts-pricing "horror stories" have occurred since last October, when the Air Force began concentrating on the problem.

"We're going to obligate in 1984 about $45 billion in 4 1/2 million contract actions," he said. "We're going to make some mistakes. But by and large people are doing a damn fine job." CAPTION: Picture 1, A $7,400 coffee brewing machine, "the first of its kind," installed in the galley of a C5A cargo plane. Department of Defense; Picture 2, Hot Beverage Unit; Picture 3, Hot Beverage Unit. Air Force repair-manual diagrams show the complexity of the coffee machine, which has more than 2,000 parts.