Need some help with this kind of flush valve.

[moonlight]

These are the armtage shanks cheap close coupled toilet flush valves. They usually have the two "foot" type push button, one large circular plate and one small white foot, they are rarely found in a concealed cistern as the button would have to be fit in the top of the unit.

You'll mostly see them in new builds and I've seen whole estates with the same problem. Trust me I did toilet repairs for 4 years on bg home care. It's the one flush valve I'd never change a washer on.. through bitter experience

The one above is the Pneumatic version not the one with the two feet.