Sending unit has weird vent thing

Call Tom at Instrument Specialties, he reconditioned my sending unit. He should be able to do it. A friend of mine swapped out the 5 1/6" to 3/8". Like post #15 said, unsolder the 5 1/6 tube, bend up a new 3/8 tube with tubing bender and solder or tig weld it back in. Get a new 3/8 end filter.
There are other people that recondition these sending units, maybe someone else can jump in with their names.