The Metrorail segments next in line to open -- the Yellow Line between Gallery Place and National Airport and the Blue Line extension to Huntington--will not go into service before July 1983, seven months behind schedule, Metro officials predicted yesterday.

General Manager Richard Page blamed a 3 1/2-month-old strike at a Pennsylvania brake factory that is holding up production of rail cars needed for the 8 1/2 miles of new track. "There's no predicted end to that strike at this time," Page told the Metro board yesterday morning. A long-term deadlock could push the opening past July.

Late car deliveries will also force Metro to rethink the late-1983 target date for opening 14 new miles of Red Line through to Shady Grove, Page said.

Construction timetables had shown the Blue and Yellow track and its five new stations opening in December this year. In recent weeks, Metro officials talked of early 1983, then settled on July 1983 as a more realistic date, since the strike at Westinghouse Air Brake Division, part of the conglomerate American Standard Inc., remains unsolved.

The strike stopped work on brake systems that the rail cars' manufacturer, Breda Costruzioni Ferroviarie of Italy, needs to complete prototype cars and begin production. Metro has ordered about 300 rail cars from Breda, the first 50 of which are needed for the Blue/Yellow opening.

That new track will bring the first Metrorail service to Alexandria and make Blue the first of the planned 101-mile rail system's five color-coded lines to be finished. Thirty-nine miles of the system are now in operation. It also will inaugurate the Yellow Line, with trains crossing the Potomac River on their own bridge at 14th Street.

Yesterday, Page raised the possibility that limited rush-hour service across the Potomac on the Yellow Line might begin before the rest of the new track opens. Commuters traveling between downtown and points near National Airport now ride miles out of their way to cross the Potomac on Blue and Orange trains near Key Bridge.

Delaying the opening of the two segments would bring some relief to Metro's pressured operating budget for fiscal 1983. Savings from delaying hiring 177 people for the new segments are expected to more than offset the continued expense of providing heavy Metrobus service in the area.

These savings would help cushion any reduction in federal operating aid below expected levels. Metro's draft budget for fiscal 1983 counts on $17 million in such aid. However, the Reagan administration is proposing aid levels of $14.5 million, or about 4 percent of Metro's proposed $365-million budget.