Is it possible for the multiport valve to malfunction and allow sand from the filter to enter the pool via return jets? I've had sand by all the return jets for the last three years. I replaced the sand 3 years ago when i purchased the house (it was a swamp when we moved in so not sure if this problem existed beforehand). I checked the laterals and they looked good but i replaced anyway just in case. I have since emptied the sand twice to make sure that i had the laterals correctly positioned and the problem persists. Any suggestions?