Vice President Bush joined Labor Secretary William E. Brock at a campaign-style pep rally at the Labor Department yesterday, a ceremony that the Reagan administration hopes will mark the revival of a demoralized agency that has been seen as a political liability.

Backed by an Air Force band and a banner that proclaimed "A New Pride" for the department, Bush called on the standing-room crowd of 1,000 employes to "reaffirm the importance of your mission and renew your pride in your colleagues."

Bush's visit underscored the importance the administration has attached to the 18,000-member department in recent months after former secretary Raymond J. Donovan became the subject of a lengthy criminal investigation. Donovan stepped down six weeks ago to fight his fraud indictment.

Praising Brock as "one of the most credible public servants in the nation," Bush said the department would work vigorously under the new secretary "to make the lives of workers less dangerous, more dignified, more productive, and more secure."

The appointment of Brock, the former U.S. trade representative and Republican National Committee chairman, was regarded on Capitol Hill as a decisive move by Reagan to restore order to a troubled agency and to enhance Republican standing among both union and nonunion voters.

Brock, who took office Monday, has moved quickly to patch up relations with organized labor, which had an icy relationship with Donovan and delivered the majority of its vote to Democratic presidential candidate Walter F. Mondale last November.

Brock's first major appointment was Stephen Schlossberg, a former top official of the United Auto Workers, who will become deputy undersecretary for labor-management relations and who is highly regarded by organized labor and by segments of the business community.

The Schlossberg appointment drew immediate fire from anti-union groups such as the National Right to Work Committee. But Brock's spokesman, David Demarest, said yesterday that Brock intends to "make a variety of appointments drawing from a wide spectrum of opinion."

Brock is continuing to mend fences with labor unions, and has a meeting scheduled Tuesday with the 35-member executive council of the AFL-CIO. It will be his second visit to the 13 million-member labor federation since his appointment was announced. "He's already been here almost as much as Ray Donovan," a federation official said.

AFL-CIO sources said that union officials hope to persuade Brock to fire Robert A. Rowland, head of the Occupational Safety and Health Administration, and to step up OSHA enforcement and speed the issuance of long-delayed federal standards to restrict toxic substances in the workplace.

Brock is expected to replace several top officials, including Rowland, a Texas lawyer and Republican fund-raiser whose financial holdings have prompted conflict-of-interest allegations, and whose failure to enact new health standards has drawn strong criticism from members of Congress.

Brock told the enthusiastic gathering of political and career Labor Department workers that the administration remains committed to developing programs for unemployed, handicapped and displaced workers.

"We have a responsibility to working people to assure the safety and dignity of their workplace, and their pensions, and their security," he said. "I am excited by this opportunity to work with you," he said, to a standing ovation.

Bush made an indirect reference to the problems of Donovan's tenure, saying that the transition reminded him of a letter a young girl wrote about a television show: "I like your show, because as soon as it's over, a better one comes on."

Brock said following the rally that he plans to decide whether to replace Rowland and other officials "by summer," and that he is also reviewing recent department actions, such as Rowland's April 12 decision not to enact a federal standard requiring toilets and clean drinking water for farm workers.

"They are trying to convince people that things will be different and better, but that will depend on his appointees, and we are waiting to hear," said Michael Urquhart, president of Local 12 of the American Federation of Government Employes, which represents the department's workers.