Metro may be able to resume its long-delayed countdown toward the opening of two new rail segments, thanks to a tentative settlement of a strike in a Pennsylvania brake factory announced yesterday.

The six-month-old strike has delayed production of 50 railcars that Metro needs to open the new track--the Yellow Line between Gallery Place and National Airport and the Blue Line extension to Huntington. If members of the striking union ratify the draft agreement, they could be back at work next week, a union spokesman said.

Metro general manager Richard Page said that the new track, 14 miles long with five new stations, might open in late 1983 or early 1984. But he stressed that delays could grow or diminish as the cars are manufactured and tested. "We won't know until we have the cars here," Page said.

The cars' manufacturer, Breda Costruzioni Ferroviarie of Italy, needs brake systems from the closed factory to complete four prototype Metrorail cars. Then it must test them, make final design changes and begin actual production. Final assembly of the cars will take place in the United States.

Metro plans to buy about 300 cars from Breda.

Before the strike, opening of the two segments had been scheduled for late this year. But in February, with no solution to the strike in sight, the Metro board formally moved to put off opening until July 1983 or later.

Delayed delivery of the cars is also expected to push back extension of the Red Line through to Shady Grove, which had been scheduled for late 1983 before the strike began. Page said yesterday that the line should be completed by that time. But "how fast we open it depends on how fast we receive an adequate number of tested cars," he said.

Union and management negotiators will meet today to work out final language of the contract, before it goes to the rank and file.