What does the front brake limiting valve do?

Older trucks (made before 1975 or so) sometimes had a front brake limiting valve that the driver could set to 'normal' or 'slippery'. When set to 'slippery', the valve reduced air pressure to the front brakes, the idea being to prevent the front wheels from locking on slick surfaces and losing steering. In practice, studies showed that limiting valves caused more harm than good — they reduced overall stopping power without delivering the intended safety benefit — and they have been removed from modern vehicles. If you drive an older truck with a limiting valve, leave it in 'normal' for all conditions. The valve is not a substitute for modern ABS or appropriate driving for conditions.