There are two simple tests to determine the problem. If you do the first one, described below, you probably will figure out the problem.
The system has two major parts, the torque valve (in the drag link) and the power cylinder (the big thing that looks like a hydraulic cylinder, on the tie-rod).
The power cylinder has a rubber-edged "piston cup" inside. When the piston cup rubber edge gets old and crumbles, it can by-pass air, which returns to the torque valve, and then out to the atmosphere. It is not unusual to have it fail in one direction only, causing the symptom you describe.
To test the power cylinder, remove the two air lines that go to the cylinder, at the point where they are connected to the torque valve. With the truck engine turned OFF, parked, in gear and chocked, raise the front wheels off the ground an inch or two. CAREFULLY put some air (from the truck air supply or the shop supply) into one of the two hoses going to the power cylinder. Listen for air coming back at you in the opposite hose. Then reverse the procedure, putting air into the other hose, listening to the opposite hose. Be careful, as the wheels will try to move left and right.
If you hear air coming back at you, the piston cup is worn and must be replaced. It costs $100 including shipping. It is pretty easy to replace. We will need to know if the diameter of the piston is 4" or 5". The standard size is 5".
Let me know what model truck you have, and how you make out doing this test, then give me a call on my toll-free number 866 - 749 - 0100 if you have any more questions.
Arthur P. Bloom, VP /Military
Inter Agency Motor Pool