While driving at road speed then taking my foot off the gas pedal the truck idle runs at 1200 rpm and approximately 55 kph until I use the brakes to slow it down or stop. After stopping, the rpms drop slowly to 650. At the 1200 rpm speed I am constantly braking to stop catching up on other trafic. I expect the brakes will wear out prematurely. The check engine light code is 507 so I have replaced the IAC valve but it mades no difference. Any ideas.