I just installed an Intelliflo VF and a few days later a new FNS 48. When the pump is shut down and I open the pressure relief it pulls a vacuum and siphons the filter water and DE back through the pump, pump suction line and into the pool. Checked with leak detect on all the joints and did a low tech hydrostatic test on the suction line. No suspected leaks. Based on what repair guy answered in the post below and my suspicions that's its a simple gravity flow issue. I'm installing a check valve on the pump discharge to filter inlet line tomorrow. Have to see after if it works or if the filter/DE water will then siphon through the return line.

Not 100% happy with the clamp fitting on the FNS. Seems I have to tighten down 8 or more turns past where the spring coils are touching to stop minute leaks around the clamp. Tapped with a rubber mallet while tightening, O-ring lightly silicone lubed. Perhaps if Deeker sees this he may have a suggestion.

I'll let you know if the check valve solves the problem, although I'm still am suspicious I have a leak.

http://ths.gardenweb.com/forums/load/pools/msg0518153025944.html

What does it look like when it is leaking back into the pool? My new pool was filled Friday, and there is some kind of white-gray flaky stuff floating around in there now.

Is that normal for start-up for the first time?

The culprit: Faulty maintenance by the pool owner (me). Turns out the filter just needed a good cleaning. The DE had started flowing back which gummed up the backwash valve, which in turn caused everything to leak.

To answer OakRunFarm: The DE going back into the pool, looked like a very light gray cloud of dust. It gathered back in around the Main drains.