By Bernie Woodall

DETROIT (Reuters) - U.S. safety regulators on Thursday warned millions of owners of older model vehicles in "Rust Belt" states to get their brake systems checked for corrosion, a warning that stems from a long-running investigation of complaints about General Motors Co trucks and SUVs.

The "Rust Belt" includes about half of the 50 U.S. states. It is where salt is used to melt snow and ice on roads, which can increase the chance of corrosion on cars and trucks over time.

The advisory stems from research by the National Highway Traffic Safety Administration (NHTSA) to determine whether rust-related problems on brake systems were a safety defect on about 2 million GM trucks and SUVs. NHTSA on Thursday also closed that investigation without a recall of the GM vehicles from model years 1999 to 2003, which included Chevrolet Silverado, GMC Sierra, Chevrolet Suburban, Chevrolet Tahoe and GMC Yukon.

NHTSA issued the wider advisory essentially asking owners to wash their cars regularly and have their brake systems inspected during cold weather if they drive in the Rust Belt. This advisory is for owners of any vehicle made by any manufacturer from the 2007 model year and earlier.

That is because a zinc-aluminum alloy widely used to coat brake systems until about 2007 was more susceptible to rust than the nylon and plastic coatings that replaced it, a NHTSA official said.

The average age of cars on U.S. roads is about 11 years, so millions of them are included in the advisory. NHTSA did not offer an estimate of the number of vehicles covered by its warning.

NHTSA said it did not determine that the GM vehicles had a safety defect in part because vehicles of similar age made by other automakers had about the same incidence of brake system corrosion.

"While we are not issuing a defect notice, there is a safety issue here and that safety issue is the combination of time and road salt creating a corrosion problem in these older vehicles," a NHTSA official said.

The regulator said vehicles in the these states are more prone to corrosion-related problems to brake lines and brake pipes: Connecticut, Delaware, Illinois, Indiana, Iowa, Maryland, Massachusetts, Maine, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont, West Virginia, Wisconsin and the District of Columbia.

(Reporting by Bernie Woodall; Editing by Lisa Shumaker)