Auto sales were down slightly in August, continuing 2017’s soft trend, but employment at vehicle and parts factories, which includes truck makers like Mack, managed to climb by 14,000 last month.

After dropping to 1,287 at the end of 2016 from 1,875 a year earlier, employment at Mack’s Macungie plant has rebounded to 1,800. To attract new workers, the company has set up tables at local job fairs, including one in May at Coca-Cola Park in Allentown, home of the AAA Lehigh Valley IronPigs baseball team.

The company has had a presence in the Lehigh Valley since 1905, when the Mack brothers moved truck production to Allentown from Brooklyn, where the company was founded in 1900.

It has endured even as giants like Bethlehem Steel, which once employed thousands of workers who turned hot metal into finished steel, vanished. And unlike many of its competitors, it builds both the truck cabs and components like engines and transmissions in the United States.

Mack is foreign-owned — by the Volvo Group of Sweden — but the parent company recently completed an $84 million project to modernize and expand the Macungie factory. Mack plans to roll out its first new truck line in about two decades on Sept. 13.

“Labor relations are good enough that they were willing to spend that much money,” said Edward Balukas, president of Local 677 of the United Automobile Workers union, which represents production line employees at Mack. Salaries start at $18.75, according to Mr. Balukas, rising to $27 after six years, and the jobs come with a full benefit package and retirement plan.