In a three line system, the carb pump has such good fuel draw, that it collapses the tank when you run the engine with the vent line plugged.
There should be NO SIDE BRANCHES in that suction line, in order to prevent air drawn in, or debris entering between tank filter and internal carb filter.
The internal carb filter is there as a last safety net. If it collects debris, it is a warning for the user he is doing something wrong.