And I have no idea what the ball in the bottom of the valve does. I've never taken one apart. Maybe thats getting forced up into the valve when you reach highway speed, blocking flow? Did you replace this part when you rebuilt the valve?

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C