For 10 years residents of the 27-square-mile-area around Potomac, covered by Zip Code 20854, have been calling for a better post office. But they want one without cars, trucks or mail carriers.

At least that's the way it must have seemed to the U.S. Postal Service last week when yet another proposed post office site was rejected, at least temporarily, amid a chorus of community opposition.

The present Potomac Post Office is a single window in the rear of a village pharmacy, and handles only letters. Residents must drive to Rockville for packages. Mail carriers drive daily from Bethesda and Rockville to serve the Potomac area.

The most recently proposed site, known as the Moran tract, is near the center of Potomac Village, beside Rock Run, a small tributary of Rock Creek. The Moran tract was one of four sites proposed two years ago by the Montgomery County Planning Board afer the board rejected four other sites suggested by the postal service.

But two weeks ago the planning board reversed itself and said a regional post office on the Moran tract poses a possible pollution danger to Rock Run, despite precautions proposed by postal officials.

And last week the National Capital Planning Commission (NCPC), final arbiter in locating the 8 to 10 new post offices that spring up annually in the Washington area, rejected the site in deference to its "sister planning agency."

However, the NCPC will take up the issue again next month, and a majority of the members of the federal planning agency appear to favor the centrally located Moran tract, as does the NCPC staff.

The site is next to the village commercial zone and within walking distance of most village stores. An 11,700- square-foot building is proposed, with 28 patron parking spaces, 40 mail carrier and 37 employe parking spaces.

Postal officials say they would leave much of the three-acre tract untouched, including all land near the creek and within the flood plain. They also would build detention tanks or holding ponds to control storm water run-off and would make the parking lot of pervious material to absorb rain-water.

A regional post office in Potomac would save an average of 42 minutes, or about 12 miles, a day on each of the 37 mail delivery routes, a saving of about 134,000 miles a year, officials estimated.

Maria Airis, representing the West Montgomery Civic Association, told the NCPC at its meeting last week, "I am not here to oppose a post office in Potomac. We need better postal service . . . but if we're trying to make the post office convenient to people" it should be closer to Rockville.

Airis said she does not oppose the site now favored by county planners, beside Bullis School about one-half mile north of Potomac Village, but expected opposition from residents of the area. The bullis site, known as the Srour tract, was once a top choice of the postal service but was rejected by county planners as an intrustion in a residential neighborhood.

mary King, secretary of the Potomac Glen Association, who lives near the school, told the commission the Srour site "is not an appropriate place for a . . . post office." Aong with other speakers, she urged that the postal service separate its "retail and industrial" facilities, slightly enlarge its window in Potomac but keep its regional facilities, trucks and employe parking outside of Potomac.

NCPC Chairman David Childs told the residents that Potomac cannot expect to have only the conveniences of a post office.

"If you have a living room, you have to have a boiler room," Childs said.