THE MAJOR LEAGUE owners let some of the air out of Washington's baseball balloon this week. They met in San Diego, traded a few million dollars worth of players among themselves, transacted their other winter business and then went home without doing a thing toward deciding whether several new teams are to be created by expanding the leagues.

Expansion is Washington's most likely means of getting a team. Last month a delegation from this city went before a special baseball committee and showed that it had the population and the financial backers to support a team of big-league ballplayers in the grand fashion to which they have become accustomed. The commissioner seemed to be enlisted in the pro-expansion ranks, and Washington's impressive presentation put it at or near the top of the dozen cities seeking teams. Baseball fans here looked with anticipation to the owners' meeting. Banners, bumper stickers and buttons began appearing all over town reading "Baseball in '87."

Unfortunately, the owners didn't share Washington's sense of urgency about the matter. They listened to the presentations of the supplicant cities and did nothing. It was disappointing, but not exactly a surprise. Baseball owners have never been among the most daring of businessmen, and their slowness about reaching agreement among themselves on major changes is legendary.

A couple of good things did happen in San Diego. Edward Bennett Williams, owner of the Baltimore Orioles, pledged himself not to interfere with Washington's acquiring a team. And the case Washington's delegation made before the owners apparently was strong enough to put it in the top rank of candidates when and if the owners ever agree to expand.

Washington's fans have been showing their good faith this year by purchasing "season tickets" for games they hope will be played here someday; they have put some $8 million into special accounts at a number of area banks. Although the news from San Diego was discouraging, this isn't the time to start making withdrawals. Patient, loyal fans who keep buying tickets even when their team is finishing at the bottom are the bedrock support of a big-league team. Here we have thousands of fans buying tickets to see a team that doesn't even exist. Sooner or later baseball will heed them.