DE LEAKING BACK THROUGH PUMP

[meanirene100]

I have an unusual problem, at least for me. when my pump is shut off, the de from the filter leaks back into the lines. when the filter is turned back on, there is a large discharge of de and air into the pool.
it is an inground 18x36/ filter de 75/ in the line chlorinator. i have backwashed, cleaned and inspected the grids for damage, changed the pumps o ring, lubed all other rings, changed pressure gauge and made sure everything is sitting level. i experimented by blocking the backwash outlet , it seemed to slow the leaching but not stop it. when i put my ear to the filter tank i can hear a slight sound of air bubbles. any suggestions on what to do next?