Some truck tractors (power units) are equipped with a bobtail proportioning relay valve (Fig. 41), which is a combination of two individual valves in a single housing. The lower portion or “body” contains a standard service-brake relay valve, which functions as a relay station to speed up brake application and release. The upper portion houses a brake proportioning valve that reduces normal service-brake application pressure when the tractor is not towing a trailer.
During bobtail operation, this valve reduces stopping distances and gives you greater control over the vehicle.
You will note that the brake pedal will have to be pushed farther to apply sufficient air to stop.