need help w/ tunze osmolator

[don.ald]

this is a second unit. i have always used these without problems.
set up my osmolator and when the water level goes below the sensor the green light goes out and the yellow light turns on then the pump (as it should), however; the pump continues to pump until the float alarm goes off and shuts down the pump. then the too high light is on. keeps reacurring.
my top off water is lower than the aquarium as it should be.
it is pumping about 4' high and the top off pail is about 4' from the tank.
its in a cube type tank sort of a biocube style so the chamber with the return pump is the lowest water level.

any thoughts?