What is this valve?

This part was connected to the top of my Hayward D.E. Filter model RG700... where the water goes out and returns to the pool. There seems to be a flap that has deteriorated and torn off which was causing problems with my pump fully priming and of course the return of water pressure back to the pool. I have removed it for now, and have drastically improved the performance of the equipment, but wonder if I absolutely need to have it. Looks like some sort of one way flap or possibly back flow preventer?? Could it be a pressure regulator? It looks like a heavy domed copper disk attached to a black flap that is housed inside this pipe and has torn off and possibly blocking the outflow back to pool. Can someone please tell me what the part is called? Can I run my pump and filter without it until I replace it, or am I causing problems to my pump by doing so. Hope my pics are good enough. If you need more specific info, please tell me what you'll need. Thanks a bunch!

.... Finally found that it is a check valve. With everything that I've read, it is mostly used if you are hooking up a heater, or have an inline chlorination... both of which I don't use. I should add that I replaced my hoses from skimmer to pump with hard plumbing, but kept soft hoses going out of filter to SWG to pool (since I'm using an Intex SWG, the hookups are easier using soft hoses. Going to work on converting those hoses to hard pipes as well when time permits).
When I look at the schematics of my Hayward D.E. Filter, it shows that the check valve would be located at the bottom of the unit where it is connected from the pump, but I found my defective one at the top of the filter head where the water flows out back to pool. Since I bought this setup used, I'm not sure if there is one at the inlet (as stated in the parts drawing) and don't want to take any chances opening that connection. Do I absolutely need to use a check valve??? Since removing it, my pump primed as it should and I have the proper circulation I needed in my pool. What can happen if I don't use this check valve?? Can I damage the filter? Does the check valve control pressure? This valve was inside a short fitting that went from 1.5" down to 1", and now that it's removed... my outflow is a complete 1.5". Is that too much pressure flowing out to the SWG and then to pool?
I'm a newbie at this pool business, so please excuse the lack of proper lingo.
 

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I don't even know what that is other than what you say it is and if everything is running smoothly, I wouldn't worry too much about it now that it's out. It apparently got stuck where it shouldn't have been.
 
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