I have a Hayward DE 4800 filter. The filter started up perfectly normal this year. I loaded it with DE and everything was working fine and shocked the pool to kill algae and clear the water. After a day of filtering, as would be expected, the pressure went up since the filter was cleaning rather dirty water...normal stuff that happens every year. The filter shut off due to the timing cycle. Here is where the trouble started. I turned the filter on and noticed that a bunch of dirty DE came blowing out of the returns. Alarmed, I shut it down. After some research, I decided to just replace the entire set of grids including the manifold since they were very old (I bought the whole assembly). During the replacement, I also inspected the the outlet elbow piping, which looked completely fine and replaced the o ring that is on that piping that the manifold goes onto. I also took apart the multiport valve to check the gasget and it looked fine. I have no air or water leaks in the system (i.e., the DE is not draining back through the supply lines when I shut the filter). I put everything back together and loaded up the DE and everything again seemed fine (DE loaded properly, nothing came out, same as original start up). Unfortunately, the same exact thing happened the next day. After the filter was off for awhile, I turned it on and dirty DE came blowing out of the returns. I backwashed the system to clear out the DE and reloaded it. It loaded fine. I ran the filter for about an hour and then shut it for about an hour. This time it restarted fine (no DE blowing out of returns). Sorry for long story, but wanted to cover everything. It seems to me that after the filter gets dirty, something is going on where at the moment it is turned back on after being off a little while DE is making its way into the return path somehow. Could it be some kind of back pressure event due to the filter being dirty? I've run this sytem for 15 years, had plenty of dirty filters that I turned on before I got a chance to backwash, never had an issue. Is it possible that a defect that I am not seeing in the multiport valve, coupled with the back pressure due to it being dirty is causing this? Or perhaps the pressure is causing the the dirty DE to push past the o-ring on the outlet piping? Please let me know if you guys have encountered something like this and/or if yuo have any ideas as to what it might be. Thanks in advance.