Not peterbuilt, but on mine it was on top of the relay valve. The bobtail proportioning valve is activated by the red knob on the dash. pushing the knob in deactivates the bobtail valve. Trucks built after 95 have anti lock brakes so some vehicles got away from reducing the pressure to the rear brakes when bobtailing. If you do have antilock removing one set of wheel sensors will turn on the fault light
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