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Hello and welcome to just answer. I value your input to my questions so I may help you better. Ok let me get this picture right. If you sit still while running it works fine? It looses the assist when moving?
Does this look like your pump? with two vacuum points to it?
this your pump
Here is the vacuum layout where those hoses to the pump should be attached check to make sure they are attached properly
the valve your looking for is at the bottom of the picture. What this is supposed to do is limit assist at higher mileage to give you more road feel, but the manifold vacumm will go down then there will be some venture vacuum. put a vacuum gauge on each port and see if that changes and supplies what it should at each port , if they do then that valve on the pump is defective.