Defense Secretary Harold Brown urged Congress yesterday to approve a new Pentagon procurement scheme for stockpiling arms and equipment that could be rushed to weak but friendly countries in an emergency without denting the U.S. military's own arsenal.

Without testifying before the House Foreign Affairs Committee on efforts to beef up defenses of smaller countries, Brown ran into skepticism as to how much the leading U.S. allies in Western Europe are doing to bolster western security in the volatile Persian Gulf region.

"What we're really trying to find out," chairman Clement J. Zablocki (D-Wis.) said, "is if they are going to contribute with their own military capabilities or if they are saying 'we're going to cooperate and you go and fight.'"

While Brown generally defended allied efforts, and said it is most important that they improve their defenses in Europe rather than in the gulf, he acknowledged that consulations, verbal condemnation of the Soviet invasion of Afghanistan, economic aid and base rights for U.S. forces may not, ultimately, be enough.

"There needs to be enough evidence of their support by some kind of presence of some allies so that it will not seem that we are entirely alone in meeting the military threat in the area," Brown said.

Brown was pressed repeatedly by Rep. Benjamin S. Rosenthal (D-N.Y.) to provide "one concrete example" that the allies -- who need gulf oil more than the United States does -- take the crisis as seriously as does the United States.

"Is it conveivable," Rosenthal also asked, "that the allies are right, that they are more perceptive than we aee and that we're taking it too seriously?"

Brown's most concrete example, the expansion of U.S. access to the British-owned Indian Ocean island base at Diego Garcia, didn't impress Rosenthal, who said we had such access before the Afghan crisis.

As for the relative seriousness of the crisis, Brown said he was not convinced that the major European allies were taking it as seriously as the United States, but they are at least showing some signs of doing so now.

Brown went to Capitol Hill to testify in support of the Pentagon's plan to allocate $851.5 million in fiscal 1981 for security assistance and credit financing to 35 countries.

He called attention specifically to creation of a new "special contingency inventory" he described as an important initiative that "merits your enthusiastic support."

The plan, Brown said, is to begin buying things such as ammunition, radios, armored personnel carriers, rocket launchers, mortars and trucks, the kinds of things that are needed most at the outset of a battle, and put them into a special stockpile for foreign military assistance.

The idea, he said, is three-fold:

It could cut down delays between the time the United States makes a peacetime decision to provide military assistance to a country and the receipt of such assistance in an emergency.

It could avoid the kind of depletion of U.S. stockpiles that took place during the 1973 Middle East war, for example, when huge quantities of U.S. tanks and ammunition were rushed to Israel.

The equipment would remain in U.S. hands and could be used for training until it is needed as foreign assistance material, or it could add to U.S. war reserve stocks if it is not needed for assistance.

Brown described the initial outlay of $46.6 million for the coming fiscal year as part of the administration's "very preliminary steps" in setting up this special inventory.

He said that a study of what and how much to buy was not completed. But he estimated, under questioning, that, over a five-year period, a $500 million extra stockpile could be built up that he described as a "reasonable investment."

The total procurement budget for all U.S. military forces in the new Pentagon budget is $40.5 billion for the coming fiscal year.