A friction ring, a part of the braking system on the 5000 series rail car. This item fell off a rail car in December and struck the electrified third rail, causing a shutdown of a portion of the Blue and Orange lines. (Courtesy of WMATA)

Metro has known for six years that some of its rail cars have brake parts that fail sooner than expected, transit officials said Thursday.

The failure was found in some of the agency’s newest rail cars in 2006. Most of the defective parts have been replaced, but 184 cars with similar parts are in service, according to Metro. The transit authority plans to begin replacing the parts — known as “brake disc hubs” — in the summer.

“We don’t want to wait and see,” Metro General Manager Richard Sarles said.

When asked why the work was not being done immediately, Metro chief spokesman Dan Stessel said the rail cars are being “inspected on a daily basis to ensure continued safe operation.” The hubs “are not due for replacement until” 2014, he said, because they are newer cars.

In January and December, brake parts fell from trains in two incidents. The December incident, which occurred during morning rush hour, shut down service along the downtown core of the Orange and Blue lines for hours. Metro pulled the cars from service and examined them for “hub failure,” the same type of problem that had occurred before.

Sarles said there were two incidents in 2006 involving brake disc hubs that developed “some type of crack.”

Deputy General Manager Dave Kubicek, who joined Metro in 2007, delivered a report to board members that said the authority had planned to replace the parts in 2009 but never did the work because it did not have the money.

Sarles, who was named interim general manager in 2010, said it “took a couple of years to go through the design analysis” of what happened, and then Metro employees were consumed by the investigations into the 2009 Red Line crash. The issue did not come up again until the incident in December, he said.

Before the briefing, board member Mort Downey said Metro knew the part wasn’t “lasting as long as it should and ought to be replaced.” The part should have lasted 35 years, Downey said, but was lasting about 10 to 15 years.

“Someone’s hair should have been on fire” when the problem was known and a decision was made to fix it, said Downey, a veteran transit official appointed to the board by the federal government in 2010.

The authority has about 1,100 rail cars in its fleet, which have been delivered in batches designated with a series number during the life of the system.

The 5000 series rail cars have been problematic from the beginning. The rail cars, made by CAF of Spain, have a history of electrical, software and wheel issues, including a problem with derailments. The transit authority spent $383 million buying the rail cars, which were delivered to Metro between 1998 and 2003.

The 6000 series cars, which entered service in 2006, have a similar hub, officials said.

On Dec. 20, a “friction ring,” a part Metro officials have likened to a vehicle brake disc, fell from a 5000 series rail car and damaged two other rail cars. Metro inspected the brakes on all of its 5000 series rail cars and pulled 16 from service to be examined for possible “hub failure.”

On Jan. 6, a friction ring fell from a 2000 series rail car headed toward New Carrollton on the Orange Line. Metro inspected 464 rail cars that have similar brakes, including the 2000, 3000 and 4000 series rail cars, and replaced 290 friction rings on brake assemblies that were showing wear.

The report Kubicek delivered Thursday says an “original hub from car 5012 fractured.” The report concludes that “replacement hubs [are] required” on the 5000 series. Metro said that 98 percent of the hubs on the 5000 series have been replaced and that the remaining 2 percent are not being used for passenger service.

The hubs failed because of fatigue, Sarles said. He compared it to a car traveling on a highway with potholes. The “tracks are not smooth,” and vibrations cause the hub to crack, he said.

“It is the way this specific hub was designed, the way it was treated and the material in it,” Sarles said.

The hubs on the 6000 series were expected to last the life span of the cars, Sarles said, but that expectation changed because of operating conditions, and the manufacturer, Knorr Brake, redesigned them.

The new hubs are “bigger, made of a different, stronger material,” Sarles said.

“We have a solution in hand,” he said. “Get in, get it done.”

Knorr is paying for the new brake hubs on the 5000 series, and Metro is paying for the labor. Sarles didn’t immediately have an estimate of the costs.

Metro is still awaiting test results from the 2000 series, the type of rail car involved in the January incident.

Sarles, who was named as permanent chief executive in January 2011, has said the system has suffered from a lack of funding and lax maintenance for years. The transit authority is amid a six-year, $5 billion capital program to implement recommendations from the National Transportation Safety Board, refurbish stations and replace deteriorating equipment.