The U.S. Supreme Court handed down its last opinions of the 1986-87 term on June 26. Except for the court's opinion in the California beachfront case, none of the final decisions amounted to much. Reporters were about to hang up their telephones and go quietly when the stunning announcement came that Justice Lewis Powell was retiring. The following Wednesday, President Reagan nominated Robert Bork to succeed Powell. Since then the noise has been deafening.

The hubbub has effectively drowned out most of the term's-end commentary we have come to expect. It was not a spectacular term, but it had a character all its own. To a remarkable degree, this was Bill Brennan's term. The 81-year-old justice had one helluva time.

This was not expected. When the term began last October, with William Rehnquist in the chair of the chief justice and Antonin Scalia newly seated, conservatives looked forward to happy times. It was widely speculated that Rehnquist would prove more effective than his predecessor in welding a conservative major-ity. We fondly assumed that Scalia would vote consistently with Rehnquist on the big ones. Sandra Day O'Connor and Byron White could be relied upon. The new chief couldwork his persuasive charm upon Powell or Stevens to pick up a swingvote. In October the prospects looked rosy.

It didn't quite work out that way. By my count, the court disposed of 155 cases in all, 146 by signed opinions and nine by ''per curiams.'' In the 151 cases in which they both sat, Rehnquist and Scalia voted oppositely no fewer than 19 times. O'Connor also left the reservation: she voted against Rehnquist 26 times. By contrast, Brennan and his sidekick, Thurgood Marshall, disagreed only seven times in the whole term. It was Brennan who proved to be the big persuader.

True, a statistical case can be made that Brennan and his liberal colleagues lost more than they won, but the figures are misleading. John Paul Stevens cast 55 dissenting votes, Brennan 53 and Marshall 52. The court divided 5-4 in 44 cases. By my rough reckoning, the conservatives won 24 of these and the liberals only 14 (six were draws). What is of greater importance is that Brennan's troops prevailed in most of the biggies.

Brennan wrote the majority opinion in the case of Alabama's state police; here he upheld a Draconian plan to compel promotion of black troopers. In early June Brennan marshaled five votes in support of Maine's plant closing law. On June 15, to the dismay of Rehnquist, Powell, O'Connor and Scalia, Brennan pushed the doctrine of ''freedom of speech'' to an outer limit in a case involving the sassing of police officers. A week or so later, Brennan pushed the boundary back again: he upheld the right of a Texas constable to applaud the attempted assassination of President Reagan.

Look at the record: Brennan wrote 16 majority opinions during the term. Rehnquist dissented to 12 of them. Brennan kept his troops together to liberalize election laws, to uphold deliberate discrimination against qualified men in employment, to expand federal law on handicapped persons and to nullify Louisiana's law requiring ''equal time'' for creationism if evolution is taught in the public schools. All in all, it was quite a year for the high court's feistiest old gentleman.

Brennan came to the court on Oct. 16, 1956, by appointment from President Eisenhower. By the end of the 1987 term next summer, only eight members in the history of the court will have served longer than he. Oliver Wendell Holmes served on the court until he was 91; Hugo Black served until he was 85. Brennan appears to be in excellent health, and he has a twinkle in his eye. The way he's going, he could beat them all.

Not long ago I ran into him as we were leaving a large party. I made my manners, and he recalled an incident I had quite forgotten from the fall of 1956. I was then editor of the Richmond News Leader, and I was opposing his confirmation.

''Do you know,'' he reminisced, ''that you were one of the very few editors who took the trouble to read some of my opinions on the New Jersey Supreme Court?''

I made modest murmuring noises.

''And do you know what you said about them?''

I shook my head.

''You said they read like pages from a plumber's manual!'' And Brennan laughed all the way to the coatroom door.