Leonard,
what dave said is completely correct, the valve controls high pressure into the cylander. To create a dead spot something has to be stopping or decreasing pressure at that point. Have you tried turning the wheels with the tires off the ground? Watch the cylander as someone else turns the wheel. What happens at the cylander and or linckages when the dead spot occurs? Does a momentary leak occur? Is something binding? Is there a kink or blockage in one of the hoses? I hope this helps.
Bill