Pressure Regulator Valve
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Lo fluid is directed through the 1-2 shift valve (318) into the Lo-1st fluid passage. Lo-1st fluid is sent to the pressure regulator valve (313) to boost line pressure and at the same time to the Lo blow off valve (407). The addition of Lo-1st fluid pressure increases the operating range of line pressure in Manual First.