Budget director David A. Stockman supposedly will leave his post soon, so it figured that he would tell us now what he really thinks about military pensions and would blast the military for caring more about retirement benefits than national security.

It is a Washington tradition that the people who run things do not express their real convictions until they are headed for the door. Packing boxes act like a kind of truth serum for the high and the mighty.

Stockman told the Senate Budget Committee that military pensions are "a scandal" and "an outrage" and predicted that he would probably get into "hot water" for his candor. It is hard to tell with this White House. You have to consider the possibility that the indiscretion was programmed -- to save President Reagan, in this case, the necessity of saying something negative about people in uniform, the only public servants he considers worth their salt.

Whatever the inspiration, Stockman's outburst puts him in a class with other departing statesmen who, with one foot on the threshold, told us things they had been denying, suppressing or ignoring. Once they have spoken the truth, they have nowhere to go but out.

The most celebrated example of the truth-told-too-late is President Dwight D. Eisenhower, who on Jan. 17, 1961, in a farewell address, warned of the military-industrial complex. During his eight years in office, he held the affections of the American people tightly in his grip and with his record of heroic war service had an unparalleled opportunity to put the Pentagon and the defense contractors in their place. But he waited until his bags were packed to issue his warning

Another famous swan song came from Adm. Hyman J. Rickover, who in his salad days fought ferociously for the nuclear Navy and who, after losing his last battle to stay on duty, announced that he was "not proud" of his role in infesting the seas with atomic submarines. In a final trumpet call of egotism, he asked to be put in charge of disarmament. His farewell smacked more of a lust for command than repentance and reform.

Former defense secretary Robert S. McNamara, who was in charge of the Vietnam war during the bloodiest years, testified that from 1966 he did not believe that the war "could be won militarily." His lips were sealed at the time and when he left office.

The Defense Department launched a vigorous counterattack against Stockman. Defense Secretary Caspar W. Weinberger called his remarks "insulting and degrading. The Veterans of Foreign Wars called him "a draft-dodger."

The Pentagon is having the best of times and the worst of times. It has no wars to fight, yet its budget is being increased. Weinberger says dissenters are unpatriotic, and the president, quoting Scripture, suggests that they are ungodly.

But almost daily awful examples of waste explode in their faces, and now they have to prove that they have a system of justice worthy of the name. It relates to their most glorious adventure, the invasion of Grenada, and it involves an admiral who went scot-free for doing something that cost several soldiers their freedom and military careers.

Vice Adm. Joseph Metcalf III, who was caught red-handed on his return plane with 24 AK47 rifles from Grenada, got a slap on the wrist, while soldiers of lesser rank had the book thrown at them for similar souvenir-hunting. They were court-martialed, sentenced to hard labor, fined, broken in rank and given bad-conduct discharges.

The admiral has strict views about what is the public's business and what is not. During the Grenada invasion, he banned news coverage. And he quashed a probe of his war booty.

But it is the business of the public and everyone in uniform to know about a double standard of military justice. The views of his civilian superiors, the secretary of defense and the commander-in-chief, can hardly be withheld.

The government loses no time in responding when it feels its interests endangered. The president saw that the thugs' welcome given Kim Dae Jung and his American escorts is bad for his pal, the president of South Korea, and immediately agreed with the curious verdict of our ambassador that "blame must be shared.

The double standard of Pentagon justice is a common interest, and the common interest is what is hardly ever defended until somebody is packing it in. As they depart, members of the Joint Chiefs of Staff will doubtless tell us that the unequal justice in the contraband-rifles cases was "a scandal."

Let's hear it now, for once.