First of all your steering system should look like the one shown in the photo. Have you adjusted the end float in the cross-shaft of the steering box? If so, there is a 'high spot' in the straight ahead position. It should be adjusted so that it just passes this freely. Have you refitted the pipes correctly? The top pipe from the pump goes to the front port on the spool valve.
With the engine stopped have an assistant turn the steering wheel in the play created by correct adjustment of the tapered screw that has the big nut on top. Observe the spool actuation. From a central (neutral) position the spool should move equally in either direction. If you do not have sufficient and equal movement of the spool in both directions you will not have any assistance. Check for any reluctance to return to the neutral position. Its possible that the adjustment might be incorrect and allowing some pressure to reach the cylinder. By revving the engine you may just create enough flow to turn the steering.
To adjust the steering the adjustable barrel should be disconnected at one end. Screw the tapered screw down until the two arms are locked together. Adjust the barrel so that the pins can just be fitted. Unscrew the tapered pin SEVEN turns. That is the initial adjustment. Nip the lock nut. Have your assistant turn the wheel back and forth and observe the movement. Look for any play in the pins/spool yoke/steering arm/vertical shaft bushes/shaft/cylinder rod anchor point in the axle casting. You are unlikely to have any axial play in the rod but 'up and down' in the casting will destroy the efficiency. The vertical shaft grease nipple should be visible from the left hand side on the front side of the axle and about 4 inches directly above the clamping bolt for the bottom steering arm.
If you don't get anywhere disconnect the outer ball joints and jack the front up. Make sure that each wheel steers freely.
Please let us know what you find.
DavidP, South Wales