1999 Ford Expedition front to back A/C

Hi, I just bought this truck and the old owner said if you clamp an a/c hose up close to the fire wall passenger side you will get a/c in the front of the truck and none in the rear and unclamped I only get a/c in the rear of the truck and the front is warm, is there some kind of valve that has to be replaced and where is it located. Thank you. Sal