The trans solenoid used with the right valve body is simply a means of diverting pressure to the reverse gear at the same time as low gear, giving the effect of locking the trans up. The release of the solenoid disengages reverse gear, sending you off down track. The trans brake does not work in high gear and should NEVER be applied unless stationary. You press it to reverse because the functionality or selecting reverse is now a combination of the shifter position AND the solenoid.
All the shifter controls is the direction of fluid and therefore the fluid pressures to each gear position. It's therefore critical that the shifter puts the selector in the right gear detent, otherwise you will effectively have multiple valves open in the valve body diverting pressure to multiple gears at once. It sounds to me that when you are in low at the shifter, you are really between low + high, ie low pressure to the low gear band = no movement of the car, until you rev it enough and produce enough pump pressure to lock up the high gear clutch drum. When you shift to high at the shifter, you are in high + neutral and therefore bleeding off pressure to the high gear drum which slows the car.
