Michael Eppers awaited the birth of his second child with great anticipation and planning. Knowing that his wife would have to undergo a cesarean section in late January, he arranged to take two weeks of vacation from his job as a letter carrier.

But two days before the scheduled surgery, his boss told him the Reston post office, one of the fasting growing in Northern Virginia, was too shorthanded to accommodate him.

Indignant, he left work anyway. After the baby was born, his supervisors gave in and recorded his absence as vacation time, provided he bring them a doctor's note.

"I guess I was supposed to hire someone to take care of my wife," said Eppers. "The shape that my wife is in, she can hardly walk. It will be another week and a half before everything's back to normal."

Eppers' boss, Richard Kohne, said his vacation request came at the worst time possible, when the facility was 14 employees understaffed because of transfers and illness. "We can't let anyone go that's not absolutely necessary," he said. "I've scheduled people six days a week, 10 hours a day and now someone has to get {Eppers'} job done after they do theirs."

Eppers' predicament is an example of the difficulties in the post offices served by the Merrifield processing center, where mail volume has climbed with residential and commercial growth.

The volume of mail at Merrifield, the hub of all regional mail delivery, increased by about 38 percent over last year, up by 1.5 million pieces a day. In contrast, the operating budget for the facility has increased by only about 10 percent, according to Fran Ford, a spokeswoman for Merrifield. This year's operating budget is $215.7 million.

Concerned that the agency is having trouble keeping pace with the demand for service, Rep. Frank R. Wolf (R-Va.) asked the General Accounting Office to investigate mail delivery in the area.

Wolf made the request after the U.S. Postal Service delivered to him a 1 1/2-page letter responding to concerns he passed on to the agency two months ago.

The letter said that "significant improvements" had been made in service and that all first-class mail was being delivered before 6 p.m. It also said the agency had hired 229 additional employees and purchased 60 new mail trucks and cars.

The congressman "was a little disappointed" that the report was not more detailed, said Edward J. Newberry, a spokesman for Wolf who said his office is still receiving six to seven complaints a day about mail delays. "I don't think he felt the response was adequate given the magnitude of the problem."

Ford said there has been a marked improvement in delivery since October, when Merrifield experienced its biggest backlog. The facility, which employs 5,600 workers, routes mail each day to 85 post offices and 55 post stations in the region.

The U.S. Postal Service, a semi-independent corporation, is in the midst of a national shakeup that includes a rate increase, service cuts, a $510 million federal budget cut and management shifts. A new postmaster general, Anthony M. Frank, was named this week.

For its part, Merrifield received orders yesterday to cut its window service by 10 percent and to stop Sunday collection next weekend, said Ford. She said the details of how the cuts will be implemented have not been worked out.

Sources at Merrifield said there have been occasional delays in first-class mail delivery in recent weeks, that some bulk mail has not arrived at its destination and that the amount of overtime that employees are asked to work has taken its toll on employee morale.

"The biggest problem still is six-day weeks," said Eugene F. Smith, vice president of the Northern Virginia local of the American Postal Workers Union. "People are getting sick more often." He said 1,200 employee grievances were filed in 1987 compared with 900 the year before.