I went down this sliperly slope a while back. It almost cost me a plane and did cost me a lot of fiberglass cowl work.

Take all the Ts out of the system. The three line fuel system is the cleanest for cowled engines. Normal Clunk to the carb. Second Clunk on the fil/defule line. Third is a normal vent to the muffler. The second clunk will use a fuel dot to plug the line when not fueling or defueling.

My mistake was to use a T in the carb line with a plug on the filler. I was ok on fueling as my fuel dot was straight to the tank, but without a clunk, just a second vent type line. I would fill until the tank overflowed into the muffler. A little messy, but OK. But to defuel, I had to use some forsceps to pinch off the line between the T and the Carb, otherwise it just sucked air from the carb and never emptyed the tank. Well that worked OKthe first day. The second day out with the plane, it stumbled once when Iran it up priro to take off. I couldn't get it to stumble again, so I took off, well almost, A stumble just as I was rotating and then dead stick with a barbed wire fence comming on fast. Lots of fixing needed. The stumble was because when I used the forcepts, I cut a couple holes in the fuel line to the carb. Under full throttle, it sucked air, and went dead stick. I've replaced the T and added the second clunk and all is well. Just keep the second clunk line and brass short. You only need the clunk for defueling, so it doesn't have to do any job other than suck the bottom of the tank when the plane is setting level.

Don

PS, I'm not a fan of pinching a fuel line with anything much rougher or harder than my fingers now.