That is the power piston/power circuit. It will often be stuck in the down position (yours looks like it is in the right position, though), When it is in the down position, it pushes down a ball in the bottom of the mast which lets more gas in and makes the carburetor run too rich at all speeds. Make certain it slides freely in the bore. When vacuum drops (in acceleration) a spring allows this piston to drop down and open the power valve. When vacuum returns this piston hopefully goes back up and lets the engine run at the normal fuel/air mix.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end