Tractor Repair: Ford 801 fuel fitting??, tractor diesel, injector pump

Question
What is the function of the brass fitting in the front of the fuel tank on the 801 power master farm tractor(diesel)? It screws into the front of the tank, has a re-circulator line coming from the injector pump, has a 1/8" line coming from the engine block and they meet at this tee type fitting. It has a 1/8" tube on the top inside the tank rising up through to the top on the inside of the tank.
Thanks Chuck

Answer
well the one at the front of the tank is a return or junction block the one at the top, the small one, if it just comes out and doesn't go to anything at the top its a vent, the little brass fitting that the 1/8 inch line is attached to is a small check valve....we also use these to prime the fuel system with compressed air