/diy/ - Do It Yourself

any plumbers here? wanted to replace the concealed cistern as the inlet filling valve didnt work properly anymore. But now the new flush valve seat leaks. but i have no idea how to get it tight as its a simple press fit and exactly the same part as before

Any plumbers here that can help me? i have a toilet with a concealed cistern that started leaking and i cleaned the inside and bought a new kit (identical to the old one, so i dont think i have the wrong fitting one). I installed the new kit but the toilet ist still leaking as the thing that goes in the wall is basically just a friction fit but i have no idea how to get it to seal properly it as far in as it goes.
>pic related