32 gas gauge sending unit, not working

[oldford]

The gauge at the dash is working. I can blow dry air in the disconnected line at the sending unit and the red liquid will rise. Put my thump over the end and the liquid will stay. Lift my thumb and it goes down. I've got the tank sending unit out and for 90 years it looks pretty good. There is a small hole at the bottom of the chamber where the air line goes into and it is not plug. What should i look for. thanks