It seems indeed that either the pump or the steering rack (or both) is faulty. I will try a DOHC steering rack that I still have lying around. In the past the steering was a lot better using a DOHC steering rack, but on full lock the tyres hit the chassis rail which is an MOT failure, so that's why I fitted the cossie rack. But I found a company online that makes steering lock stops for the Sierra, so with those I should be able to use the DOHC rack again. But the car is off the road for the winter, so when the temperatures go up I will have a try if the DOHC rack solves the problem.