You need to measure pump outlet pressure when turning the steering at idle. it could be the pump has a worn rotor, or it could be weepy rack seals. I could do this, but would need to make up a pressure adaptor, which I would have to charge for. To be honest the cheapest, quickest and easiest option is to take a risk and try a new pump. I would also make sure the gauze in the bottom of the fluid reservoir is TOTALLY clean and clear, (wash it out with brake cleaner, thinners, or petrol), and that the O ring and the pipe for the SUCTION side of the pump are perfect with NO air leaks allowing the pump to suck air. If it were to be doing this it would usually be noisy though. Is it noisy? Has it lost any fluid in use?