Last weekend was a grisly one for defending baseball champions. The Pittsburgh Pirates, Baltimore Orioles and Cincinnati Reds all entered their weekend series Friday with plausible expectations of making dramatic moves from just off the pace in their division races. What good is being the champ if you can't scare the bejabbers out of the pretenders to your throne with a late-season blitz?

Instead of sounding the "charge," somebody played "retreat."

By Sunday night, all three were in grave danger of joining the California Angels in the dross heap marked "Past Champs." The league titlists from Pittsburgh and Baltimore each found themselves five games back in the loss column, while the Reds now are 4 1/2 in arrears.

The Buck visited first-place Montreal with high comeback hopes against a young Expo team that always has feared them. Instead, they lost two out of three. The Reds hosted front-running Los Angeles, only to be swept three straight on their own Riverfront rug. And the Orioles thought they would make-up ground when they played Toronto while New York went to Boston. Instead, the Yankees swept four from the limp paper giants of Fenway, while the Birds split. So much for miracle comebacks.

Chances are that at least one of the three will rebound enough so that its followers -- who now are abondoning hope -- will get mightily excited once more. The universal tendency in baseball is to jump to conclusions, ignoring the rock-bottom fact that the sport is a game of drastic streaks, any one of which can reverse itself.

Nevertheless, the Bucs, O's and Reds all have reached the point where simple arithmetic is their worst enemy. They may make one last move; they may cause a final fuss. But they are unlikely to get over the hump and reach first place.

While defending winners have floundered, this has been an excellent September for the return of past dynasties which looked like they might have become extinct. It now is possible that our old friends -- the Yankees, Royals, Phillies and Dodgers -- will be the four clubs in the '80 playoffs -- just as they were in '77 and '78. In fact, three of those four -- excepting only the Dodgers -- also were the division champs of '76.

Yet all four of those late '70s powerhouses have had such sudden face lifts and personnel changes that they hardly seem like the foes who met just two years ago.

The most dramatic conclusion that can be drawn for these trends also is perhaps the simplest one. The old champions of '77-'78 fell from the top -- in large part -- because they stood pat and refused to retool while at their peak. Only after they had fallen did they patch up the problem areas that they knew existed ever when they were waving pennants. Now, in '80, the same progression has repeated itself. Every division champion of '79 had at least one weakness that was common knowledge to its fans. None made a move to correct it.

The world champion Pirates knew that their starting pitching was thin, especially after losing Bruce Kison to free agentry. And they knew that their power was old -- especially Willie Stargell and Bill Robinson. But everybody knows you don't break up a winner. So the Bucs made a few moves. Now, they have an emaciated starting staff, a worn-out bullpen and a pair of decrepit oldsters in Stargell and Robinson.

The American League champion Orioles knew that their bullpen was weakened by losing free agent Don Stanhouse and that they needed a righthanded power hitter as either a designated hitter or an every day outfielder. They also knew that they had at least one extra starting pitcher troublesome Dennis Martinez, whose reputation outside Baltimore was much higher than it was among the Birds who watched his 15-16 season.

Ah, how perfect hindsight always is. The season, the Birds have lacked right-handed punch and never have had cause for genuine confidence in their bullpen. They still may win nearly 100 games and finish with the thrid-best record in baseball, but in their heart of hearts, many a Bird must wonder what Martinez would have brought on the trading block then, as opposed to now. w

Perhaps nothing could have saved the California Angels from the horrible injury jinx that has haunted their franchise throughout its existence. If the San Andreas Fault ever opens, it probably will swallow up the Big A. Nevertheless, the Angels knew last November, as soon as Nolan Ryan defected, that their starting pitching, which had been 21st in ERA in baseball, was bad to begin with and in danger of becoming awful. But the Angels talked bravely about how they only needed a pair of 8-7 pitchers to replace the 16-14 Ryan. Now, California is saddled with a staff so atrocious that no one knows how long it will take to rebuild a contender.

In the NL West, the Reds haughtily continued to disdain the free agent market, assuming that their nucleus of Hall of Famers, plus a few farm products could overcome any number of personnel loses. But this was the year that Tom Seaver, Dave Concepcion and George Foster all sagged together. Goodbye, Cincy.

Conversely, it is just as fascinating to observe that the old champs of the late '70s have returned to power because they finally solved the problems that they recognized long ago, but could not or would not, solve when they were kingpins.

The Yankees always were the most honest with themselves -- ruthlessly honest, as befits owner George Steinbrenner. He saw the threadbare starting staff of September '78 with Jim Beattie, old Catfish Hunter and even Dick Tidrow. That's why Tommy John and Luis Tiant arrived in '79. It wasn't enough.

Now, the Yanks have a starting fiveman staff -- Gaylord Perry, Tiant, John, Tom Underwood and Rudy May -- none of whom was in pin stripes in 1978.

Similar cures have been effected elsewhere. The Royals needed a new manager and a relief stopper. They got Jim Frey and Dan Quisenberry. The dodgers needed wholesale pitching shakeups. When the buying, trading and winnowing ended, the Dodgers had discovered Jerry Reuss (17-5), a revitalized Don Sutton and a hoard of new relievers, some of whom do their job some of the time. The Phillies always needed a vastly changed team attitude, but they wouldn't make the logical move from Dutch Uncle Danny Ozark to Loud Uncle Dallas Green until the team failed to win its division.

Team transformations are not this simple, of course. They require chapters of analysis, not epigrams. Nonetheless, a trend does run through the division races of '80. The repeat winners of the '76-to-78 years held their cards patly until their time in the winner's circle ran out. Perhaps they were fortunate to stay afloat so long.

The division champs of '79, it now appears, took the same approach, delaying painful moves.

After the unexpected results of last weekend, their fortune does not appear to have been nearly so good.