Good afternoon, We have had considerable experience with these trucks and the symptoms you are describing. The S150A were notorious for poor break-away [from a standing start] steering. We have found the following things will improve the steering.

1 replace the belt with an OEM Hyster belt [no aftermarket belt works as well. In fact we stock one at all times!]
2 Replace the pulley with an OEM pulley, if even slightly worn.
3 Increase relief pressure on the power steering pump from 1200psi to 1800psi. Relief valve is in the side of the rear cover of the pump. Remove the roll pin and plug to access the relief valve, remove one of the thinnest shims, re-install and test with gauge. Repeat process until you get desired pressure.
4 Finally, if all else fails, replace the steering tires [tyres] with the hardest compound tire [tyre] you can get. There is a lot of weight on the back axle and softer tires will "flat spot" and grab and smooth surfaces.

We have seen dozens of these over the years as we work on the components off them [Pumps, orbitrols, valves, axles, etc.]. When we get a call regarding poor steering, the first thing we do is send the customer a new OEM belt. It solves the problem 50% of the time. From there, you may have to get aggressive.

Hope this helps. Regards, Keith Williams - Aztec Hydraulics Inc.