Hy jdchevy, welcome, if I'm remembering correctly a spring pushes the shifter into low range when there is no vacuum. Most of the dash controls I have seen say "Park In Low" on them, because when the vacuum is gone the spring will try to push it into low range anyway. You should have check valves in your system, when you open the throttle when driving along your engine vacuum will drop and that may allow the vacuum in the two speed shift diaphragm to drop to the point that the spring will push it out of high range. You should test the integrity of the diaphragm before you go too far trying to get the shift system to function, hope that helps