While postal workers are considering walking off their jobs, six private letter carriers here are seeking to keep making their appointed rounds in the face of U.S. Postal Service opposition.

The six work for P. H. Brennan Hand Delivery, Patricia Brennan, sole proprietor, and they are the only private first-class-mail carriers competing with the Postal Service.

They are not loved by the Postal Service, out of whose mouth, according to the government, they are taking bread, and which, by act of Congress, has a monopoly on such service. But their customers love them.

More than 175 of their 357 clients have written letters in support of the Brennans to be used with their legal appeals. The letter say over and over that the Brennans are faster, cheaper and more dependable than the postal service. One letter adds that they are also "a friendlier service."

Last week, in the face of a possible postal strike, the Brennans' service won another in a series of 11th-hour reprieves, which prompts Brennan fans to comparisons with the movie serial "Perils of Pauline." On a technicality, the service was allowed to continue delivery until the 2nd Circuit Court of Appeals once again considers its case - which could come as soon as tomorrow.

J. Paul Brennan, husband of the proprietor and, shod in blue Pro-Keds, one of the carriers, was greeted with messages of encouragement in office after office on his morning rounds Friday. But uncertainty keeps the Brennans and their customers wondering.

"See you Monday," people called after him. Then they changed it to a question. "It looks good at least for Monday," Brennan replied.

The Brennans only serve a roughly two-mile-square section of Rochester called the Inner Loop, but they are providing what the Postal Service once promised but failed to deliver - same-day service.

A letter picked up by P. H. Brennan in the morning arrived that afternoon. Each regular-size envelope costs 10 cents regardless of weight, while manila envelopes cost 25 cents. Customers upon customers explain that their postal costs have dropped by a third or more since they started using Brennan. Customers are billed at the end of each month.

Not only is it cheaper, but Brennan carriers will wait at court clerks' office for receipts, will correct addresses, will take time to search their mail bags for a letter from a customer who suddenly wants it back, will redeliver U.S. Postal Service mail that has been taken to the wrong office and will deliver live turkeys.

At least, they have delivered one live turkey, which was sent by one lawyer to another. "It was by way of a comment, I think," Pat Brennan said. She recalls that people in the elevator kept their eyes fixed forward as though they weren't sharing the ride with a fluttering and clucking bird.

P. H. Brennan has never lost a letter or missed a day of its five-day-a-week operation since it opened March 22, 1976. During a couple of snow-storms, Postal Service vehicles couldn't move, so their mail didn't either. Brennan carriers made their rounds.

One tribute came from a court office who had to settle a dispute over the date a document reached his court. He looked at the Brennan cancellation and decreed that their service was so reliable he would accept that as the delivery date.

The cancellation makes the Brennans a center of some philatelic activity. Some customers save envelopes and sell them to stamp collectors, while the Brennans have had visits and letters from many other collectors.

Their green-ink cancellation simply carries the company's name and the date. The Brennan logo, however - used on stationery and bills - shows a mail plane stuck in a tree.

With their employes, Charles Symington, Maureen Wedow, Todd Bludeau and Bill Hiebler, the delivery service handles more than 2,500 letters a day. There were close to 5,000 Friday.

The Brennans are quick to point out that there isn't a lot of intelligence needed to deliver mail.

"You got to have feet. You've got to be dependable. And you've got to have a lot of money to pay a good lawyer," Pat Brennan said of the requirements for anyone who might seek to emulate her.

She is pessimistic about her future. "I think we're going to lose. I don't think the courts will allow us," she said.

The Brennans have been taking $176.55 weekly from their business, Pat said, plus any surplus in the till at the end of the month. Sometimes there is none. The Brennans depend on other income - from investments - to keep them in food and mortgage money. Their employes are paid $150 a week.

The civil suit brought against them by the government seeks to close P.H. Brennan for violating the private express statutes.

The last person who tried to compete with the postal Service, Robert E. Black of Pittsburgh, Kan., was denied a hearing by the U.S. Supreme Court without explanation March 19 and is out of business. He lasted only five months.

The Supreme Court, according to Black, the Brennans and their lawyers, has never examined the issues involved.

Arguing for the Brennans, attorney James Hartman claimed that the private express statutes violate the 10th Amendment. Their interpretation of it is that, since the constitution does not specifically mention mail delivery, the federal government has no right to impose a postal monopoly. (However, the Constitution does give Congress the power to establish "post offices and post roads.)

The government argues that downtown routes like the Brennans' are the most lucrative and that a proliferation of similar services would endanger the Postal Service's future.

The Brennans operate from a $65-a-month basement office, and are able to offer their employes only one fringe benefit - a Friday afternoon session to discuss the week's events over beer. It's called "the corporate meeting."