I have a 1948 8n tractor with a marvel/schebler carborator and I have a replacement carborator TSX-241 B. Both carborators leak fuel from the bronze filters that's on the bottom of the bowl next to the drain bolt.

Why? And how do I stop this?

Get yourself a 2' piece of clear hose and a barb fitting. Attach hose to fitting, screw into bottom of carb, hold end up towards the fuel tank. Turn on fuel. Level inside the hose will correspond to the level in the carb.
Is it above the bottom 1/2?
Probably.
You can try starting the machine, Turn off the fuel and let it die. The float will drop and once you turn the fuel back on an debris in the float circuit might get dislodged and drop out.
You can also try smacking the side of the carb with a plastic handle screwdriver to vibrate it out.
Chances are it only leaks when the engine is off and then you can contact
RedrockMfg and get an in line electric fuel shut off solenoid then install that right at the glass bowl.
I have installed those in a few tractors.
YMMV.