Transportation Secretary Brock Adams said last night that he will release millions of long-delayed dollars for Metro construction, thereby removing the largest bureaucratic obstacle to completion of Metro's planned 101-mile subway by 1990.

Adams' commitment, in a letter to Metro board Chairman Jerry A. Moore Jr., came in response to a new Metro proposal that speeds up by three years completion of the Metro line through Anacostia, something the federal government has strongly encouraged.

"This is the best news from the federal executive branch for Metro in three years," Metro General Manager Richard S. Page said last night. It was about three years ago that the federal government first ordered a restudy of uncompleted, unfunded sections of Metro and began to delay approvals of construction funds.

Since that time the restudy has been completed and local politicians have reaffirmed Metro's full system as cost-effective. Adams has committed the federal government to the "goal of supporting the 101-mile system," but has kept most of the federal money in the bank.

This issue has been what lines were to be built with about $1.2 billion of available federal funds. That money is a federal credit to the regional account that was gained when several urban freeway projects were canceled.

The problem is that the Metro project would still need about $1.7 billion more to be completed. The House of Representatives is scheduled to vote Monday on an authorization bill for the $1.7 billion, although it could not be spent before fiscal 1982.If that vote fails, Metro will have to seek the money from regular federal transit programs, a much tougher course.

In the meantime, the large scheduling problems have been resolved. The new schedule assumes approval of the House bill.

Under the schedule, full subway service would begin on the following Metro segments by the following dates:

The 14th Street Bridge Metro connector from the Pentagon, through L'Enfant Plaza to Gallery Place, mid-1982.

The Orange Line, from Ballston (in Arlington County) to West Falls Church, early 1984; from West Falls Church to Vienna, mid-1985.

The Yellow Line, from King Street in Alexandria to Van Dorn Street in Alexandria, late 1984; from Van Dorn Street to Franconia/Springfield, mid-1985.

The Green Line, from L'Enfant Plaza to Anacostia, mid 1985; from Anacostia to Southern Avenue, late 1985, and from Southern Avenue to Rosecroft Raceway, late 1986.

The Green-Yellow Line, from Gallery Place up Seventh and 14th streets to Columbia Heights, then through Fort Totten and Prince George's Plaza to Greenbelt, early 1990.

The Red Line, from Silver Spring to Glenmont, late 1986.

Those segments total about 40 miles of Metro; 60 miles has already been fully funded and 31 of those miles are in operation, with the rest under construction.

Adams said in his letter that "we are prepared to use your schedule as the basis for approval of the [freeway] transfer funds," which would assure federal funding for 4 1/2 years. Adams imposed three conditions:

Metro must sell $76.5 million in real estate that it purchased with federal assistance and that the federal government considers excess.

Metro must pursue cost-cutting construction methods, including the elimination of vaulted arch stations underground. Additionally, Metro must review its plan to put the Yellow-Green Line through Fort Totten Park in a tunnel. That is a potentially explosive issue in the Fort Totten neighborhood of Northeast Washington, where Metro's original plans called for a cheaper elevated line.

Metro and the federal government must reach a final agreement on how to retire the interest and principal on $1 billion in revenue bonds sold to raise construction funds. An agreement in principle on a draft already exists.

Adams is silent on the House appropriations bill other than to recognize the need for a new appropriation. That is a shift in Adams' position of a year ago, when he overtly opposed separate legislation for the balance of Metro's construction funds.

Adams also suggested that Metro's budget anticipates too low a rate of inflation. The budget assumes a 6 percent annual increase in costs. The total cost of the Metro system, as presently budgeted, would be about $7 billion, excluding interest.

Local money is already available to match federal funds for the next two or three years. Beyond that, however, new local or state sources must be found and approved.