Under pressure from the federal government, General Motors is recalling more than 3.4 million big pickup trucks and SUVs in the U.S to fix a brake problem.

The recall covers the Chevrolet Silverado and GMC Sierra 1500, 2500 and 3500 pickups from the 2014 through 2018 model years. Also included are the Cadillac Escalade from 2015 to 2017, and the GMC Yukon and Chevy Suburban and Tahoe from 2015 through 2018.

GM says that as it ages, the pump in the power-assist brakes can put out less vacuum power than needed, increasing stopping distance and the risk of a crash.

The recall comes after the National Highway Traffic Safety Administration began investigating the problem in November of last year.

Dealers will recalibrate the electronic brake control module at no cost to customers. Owners were to be notified starting Sept. 6.

NHTSA, the government's road safety agency, began investigating the problem last year after getting 111 complaints from owners of poor brake performance. At the time the agency had nine reports of crashes that had caused two injuries.

GM says the problems occur rarely and mostly at low speeds like driving in parking lots. Even with the problem, the brakes "continue to function and exceed the requirements of the appropriate federal motor vehicle safety standard," GM spokesman Dan Flores said.

The vacuum pump is lubricated with engine oil that flows through a filter screen. In some of the trucks, oil sludge or other debris can clog the screen, reducing oil flow and causing the pump's vacuum output to drop, Flores said.

The trucks, he said, have a secondary power brake assist system which works when there are problems with the main system, but it is limited at lower speeds. The reprogramming will activate the secondary system faster.

GM isn't replacing the pumps because pump failures are rare and already are covered by an extended warranty, Flores said. "This remedy will improve brake performance in any situation where brake vacuum drops," he said.

The recall comes 10 months after NHTSA opened its investigation. Asked why it took that long to do the recall, Flores said only that the company has been cooperating with NHTSA and providing data.

Drivers could feel a vibration in the brake pedal or a change in pressure required to push the brake pedal if their trucks have the vacuum pump problem. If the problem surfaces, owners will see a message on their dashboard telling them to service the brake assist system, Flores said.

Although the recall is a large number, the cost will not be high enough for GM to report it to the Securities and Exchange commission as an event that will materially affect earnings, Flores said. He said he didn't know the cost.