A bobtail proportioning valve will reduce braking to the drive axle brakes by up to 75% when unhooked from the trailer.
When a semi-trailer is removed from the truck, the truck is called a bobtail.
To prevent the rear wheels from locking up, the bobtail proportioning valve redirects most of the braking to the front steer axle brakes. This action helps to maintain steering control when there isn't any weight on the rear of the truck.
Owing to the fact that most of the braking is being done by the secondary system, the driver may find that she has to push harder on the brake pedal to engage the brakes.
Nota Bene: tractors are designed to haul trailer, particularly loaded trailers. These trucks handle "very" differently when there isn't a trailer attached. Reduce speed, increase following distance, and start braking earlier; use a deft touch on the primary controls.