Toilet with built-in element is leaking / constant trickle

Good morning everyone,

we have had the problem for some time that water runs quietly but steadily in our toilet, although supposedly everything is closed.
So I thought, it can’t be that difficult, so off to the hardware store.
As the first recommendation from the hardware store, replacing the seal for the flush valve was mentioned as a solution.

So I invested 2.90 euros and after more or less a lot of fiddling, replaced it.
The first 2-3 times it worked, but then the trickle returned.
Then I ordered and replaced the following flush valve. Again, nothing.

What else could it be?

As an idea, I possibly have dirt or lime where the valve should seal.
But how do you get there and with what do you clean it? It’s all quite tight and built-in and you can’t see anything.
I had already held my GoPro action cam in there once, but it didn’t help much.

I’m afraid if I call a professional, we’ll have to renovate the whole wall/the whole bathroom afterwards. ;)

I know that from our old house and had to try several seals until it worked. Apparently, there are several of them.

Possibly also a small speck of dirt in the flush tank, which slides under the new seal and thus allows water to run again

I know this from our old house and had to try several seals until it worked. Apparently, there are several.

At least I have already tried two. The new seal from the hardware store and the new one that came with the lifter. So I think that’s not the problem. But maybe I’ll get some more. It’s reasonably priced anyway.

Possibly also a bit of dirt inside the cistern, which slips under the new seal and lets water run again

That’s also my suspicion. Do you maybe have an idea how I can clean that? The toilet brush doesn’t fit in. ;) I had thought of a hose that you insert into drain pipes and then connect to a high-pressure cleaner. I have one, but it would probably flood half the bathroom. I’ll try to go in with a cloth. Maybe you can reach it.. Thanks anyway to both of you.

Since it is not a pneumatic box, the lever could simply be too long. Either screw it in further or shorten it a bit. That could be all there is to it.

Also thanks to you Mycraft. Ideas are always welcome. I will look into the matter again and report back...