Man, after you took it apart and cleaned the seat, it still drips? That valve has a mind of its own. It's like it's mocking your DIY skills. You might have pissed off the boiler gods or something. Sometimes these things just decide to be drama queens until you replace them. Home repairs never go easy, do they?
Ask if he matched the washer material exactly. A stiff one won't compress like the old soggy one did. Found that out after three tries on my own kitchen tap. At this point, the valve probably enjoys watching you suffer.