I can only show you in pics since I may have the terminology all mixed up as I am a pool novice.
I have a much older Hayward vac which looks like this:
Amazon.com : Hayward 2025ADC PoolVac XL Automatic Suction Pool Cleaner for Concrete Pools : Swimming Pool Suction Cleaners : Patio, Lawn Garden
My skimmer has 2 holes in it leading to the pump. One of them has something which looks exactly like this.
It seems the purpose of this device is to close up and so the vac can suck water in.
Lately its been opening up inadvertently causing the vac not to work and a rush of water into the opening creating a sucking noise I can hear from far away inside my house. I checked and the springs are still tight. Seems to have three settings. I thought moving it to the other three extreme setting would prevent it from opening to let in water. IT still opened up despite the change in setting. While tinkering with it the red latch came off. I left it off and it seems to be doing OK so far (30 minutes ago). In trying to find out what the device is for I came across that is is for Barracuda vacs. As I said I have a Hayward vac. Should I replace it with what is below? BTW the device has been there since I bought the home 5 years ago. Off and on have had problems with it.
Should I replace it with this? Do I need a new one of the above despite the fact its would be OK if I put the red plastic piece back on? Why is it opening? BTW the pool water level is OK IMO.
Amazon.com: Hayward V094P Automatic Pool Cleaner Regulator Valve for All Hayward Suction Cleaners: Patio, Lawn Garden