what 'fills half way'? the carb bowl? an inline filter? the glass bowl filter on the pump? ....
is the 2nd pump a new one or another old one?
has the carb been rebuilt? is the float set properly?
is your tank clean, or is there sludge in it that gets sucked against the tank outlet and stops/slows the flow?
have you checked any rubber sections of fuel line for deterioration, like maybe slightly leaking weather checks or collapsing internal layer?
lots of possibilities to investigate

Bill


Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics