Diaphragm in the rocker cover

Hello friends. My car is 307 1.4 hdi. There was an oil leak in the engine, oil was coming over the turbo. I showed it to the master and he said that his diaphragm needs to be changed. He said that all of the carburetor caps need to be changed for him to change. Do you have any information, can't the diaphragm change alone?

Hello, teacher, what's the matter, all of them have changed, master has flown. I think replacing the gasket on the cover is enough. Can you upload a photo of that piece? what the master said

No, sir, it is leaking from the gasket. There is a connection to the turbo. I think you mean the diaphragm. He said he had to go and change. He said that for him to change, the whole cover needs to be changed.

I wondered what it was like, and it's something I've heard for the first time. I did a little research and there were those who had it done in the appropriate way, of course, I couldn't find an explanation about how it was done, unfortunately.