You probably have a leak somewhere.
Is the filter filling with air when the system is off?
If so you need to pressure test the system to find the leak, unless you can hear the air entering when it is off.
This could be a pipe, a seal, air relief vavle, ect.

Your system is admitting air somwhere between the front of the pump and the top of the filter.
It could be the o-ring under the pump lid or the pump's male adaptors, it could also be the o-ring between the two halves of the filter or the one under the pressure gauge.
Start up the pump and achieve a full prime then turn the pump off and on rapidly, look for water at these spots. If you don't see any look between the wet end and dry end of the pump and beneath the pump too, it could be the pump's o-ring or the seal. It could even be the o-ring under the handle of the Jandy valve.

See ya,
Kelly

Thanks for the responses. I did notice that if I turned the backwash valve to
the boost/bypass position I was able to inhibit the flow of water out of the
filter tank into the pump. In the next couple of days I'm going to re-seat the
"O" rings on the filter. I think the ring around the base of the pressure gauge is
definitely a lead candidate for the cause of the problem. What type of "O" ring lube
should I use ? I think I read somewhere that a petroleum based product would damage the
rubber.
Thanks again.

Teflon or silicone based like Magic Lube or Jacks. Jacks is good for smaller ones as it's thick. Big rings Magic, easier to spread.

Scott

Well today I took my filter apart and in the process of putting it back together I used lube on all the "O" rings.

I ran the system and experienced no leaks. After shutdown I still had DE flowing slowly back into my pump housing. I then decided I would add an additional return line to the pool just to make sure I was not creating a backpressure problem on the return side. After running the system a while I shut it down and once again DE began flowing into my pump.

My next thought is to place a flap style check valve between the pump and filter. I was always under the impression that the design of the filter and the Hayward Selectra-flo valve would keep the DE from moving backwards.
Could it be that the valve needs cleaning or maintenance?
Does it serve any sort of check valve function to begin with?
Any ideas would be appreciated.

The MPV has no check valve. Putting a check between the pump and filter will do it for you.

Any chance the pump is below the water line but the top of the filter isn't?

Scott

The suction plumbing is undersized and the maximum HP on that size pipe should be a 1HP. The restricted flow in the suction line will cause "cavitation" literally pulling the oxygen out of the water. This can accumulate in the filter as trapped air, (as could a suction side air leak as mentioned by others). The compressed air expands when the pump shuts down traveling the path of least resistance back through the skimmer. With the reversed flow the DE is carried. This can be dangerous, not just inconvenient. I have seen skimmer lids blow 30 feet into the air, and the compressed air places the filter at risk of explosion if it get too high or if the pressure is not relieved prior to dismantlement. The switch to a smaller pump, (1 HP) would be only a very small decrease in flow as most of the pumps work is now done creating pressure, not flow. The smaller pump would eliminate cavitation as well and prolong the pump and system life. Best to you! : )

My filter used to do this because there was a leak in booster pump for the polaris sweep. As stated above a check valve between the pump and the filter should solve this. Somehow there is air getting in your DE filter.

My guess is maybe the gasket on your DE is not seated correctly or the air relief thing at the top of your DE is leaking slowly.... but i could be wrong. The filters in the DE would have nothing to do with this.

Thanks for the ideas. Yes the pump and filter are above the waterline. They are on a pad that sits approx. 20 inches above the pool deck. As to the restriction on the suction side I will first experiment by opening all the suction valves. As of now I'm restricting the skimmer(2") to create the suction needed to operate the Polaris G4 pool cleaner.

As another thought.. it occurred to me that the filter is designed to allow water in by the way of a pipe that goes straight up to the top of the filter housing. It has an "o" ring at the top and the entire grid assembly slips down over it. The pipe must be designed to act as a "stand pipe".
The only water than could enter it would be the water above the entire filter grid assembly and that's only if the "o" ring was leaking. I'm going to see if I can replace that ring today. Has anyone ever experienced a problem with this particular ring?

poolgenius, What are you talking about 2" suction line being undersized on a 2hp pump. I think you need to go back and learn hydraulics again. 1hp. You need to rethink this as well. What size filter does he have first off. What size is the pool? How far is the run from your equipment to your pool. These things all contribute to the size of filter and pump that you use. Please before you go and change all your equipment as suggested lets find out what the problem is. I am sure all equipment worked fine before they were moved.
As far as the stand pipe o-ring I recommend replacing this as well I usually replace them every time I disassemble a de filter maybe I am over cautious but it works for me. I think someone suggested taking a look at the spyder gasket another good idea. And while you have the filter apart take a look at the manifold on the filter as well These can get cracked easily by overtightening and sometimes by just shear age. If this has happened you normally get de returning to the pool though not the skimmer. But again I would start off looking at the obvious before I would make any drastic changes.

Good luck,

David

Today I removed the multi port valve and checked the spider gasket (the one that looks like a wagon wheel). It seemed fine and I reinstalled using lube on the "o" ring around the valve cover as well. Unfortunately I'm still getting the backflow. Actually the word backflow may be misleading because there is not a large scale movement of water traveling backward, it's just a slow moving white cloud that takes a few minutes to develop in the pump and then a few more minutes to make it's way out of the skimmer.
Presently the system is fairly streamlined with a relatively short (15 foot) run of 2" pipe to the skimmer and about the same distance of 1.5" pipe to the pool cleaner. The return side is 2" pipe (about 20feet) that runs to my spa wall and a 1.5" pipe about 6 feet long that
runs into the pool. The pressure gauge is showing about 7psi while running and none of my above ground connections leak when the system is running...
If I can locate an "o" ring for my "standpipe" at the pool store I will replace and report back.
Thanks for all the ideas.