SIAMP inlet valve mystery

[neate]

My inlet arrangement in the photo is filthy and probably needs replacing.
1. But to satisfy my curiosity what is the function of the circular fitting with hole in the middle?
2. And if I decide to replace the washer within fitting, what is it called?
Thanks in anticipation.

 

[Rodders53]

No idea what that top bit is.

But the inlet diaphragm washer https://shop.siamp.co.uk/diaphragm-washer-5 is replaced by unscrewing the big horizontal nut with the lever arm.
Siamp have videos https://shop.siamp.co.uk/videos
See the one at top of the list.

Just did one of my toilets the other day as it was filling very sloooowly and making my rainwater harvesting pump cycle on/off repeatedly.

 

[neate]

Thanks for the links Rodders53.
Currently as well as you there have been 79 views and no one has explained the function of that circular fitting with a hole in the middle. I am surprised!