I have been through this before too. Did you install the gasket under the new seat when you installed it? BTDT stupid I know but something to think about.
Other that that look inside your fuel tank... is it clean? I installed a cheap filter inline between the carb and the tank. It works much better at getting crud than the sediment bowl. The filter can cause its own problems of airlocking the fuel line if you run out of gas but the end result of a carb not leaking (once its cleaned again) is worth it.