Dude when I moved in to my house about 2 years ago I had the same problem and found answers in forums like you are doing. Fortunately enough I have teh exact same filter and multiport valve that you have. I can confirm you do have an American Products DE filter, and a hayward sp710 multiport valve. Now about the filter each grid cannot be broken i think there is a total of 8 grids with one small one compared to the others. (I may be wrong on the amount of grids, it may be 7 with one small). I had the same problem with the sand looking stuff at the bottom of the pool. Its not sand, its that crud that you cleaned off. Inspect each grid and if you notice any breakage in the grids then it needs to be replaced. It could be costly if you replace all at once. I replaced about 4 at 15 dollars a pop. That fixed the sand problem. If you dont replace the grids you will be vacuuming up that sand for a while. After everything was replaced and fixed then you need to seal everything up making sure there are no leaks and add DE through the skimmer (slowly) while the pool is running in filter position at the multiport valve. After this is done then you should be good to go. If you notice significant water loss and cant find the source then try the spider gasket inside the multiport valve. Right now I have a leak on the multiport valve but its a very small one and it is located at the plug that goes from the outport of the filter to the multiport valve. The multiport valve has 2 pipes going to the filter, its the bottom pipe that has a plug underneith. Hope this helps. If you have any more questions feel free to ask.