Fuel problem - help please

Hi All
I have an '83 gtv6 which I am currently part-restoring and before I started on bodywork etc I wanted to ensure the car was running correctly with no major mechanical problems. However, I seem to have a problem keeping the car running -it will start and run hot or cold but only for about 5 - 10 mins and then the fuel pump starts making a squealing noise and then the engine cuts out. I have done the obvious checks ie changing the fuel pump/filter and also the fuel lines. My experiments and checks have pin-pointed the problem to the return fuel line from the regulator to the tank. The fuel line itself is clear so the problem seems to lie from the union under the tank and into the tank.

Can anyone tell me what happens with the return into the tank? Apparently there is a valve and I have also read in previous threads that there is a charcoal filter? Any help would be greatly appreciated as this is the final thing to get the car running spot-on.
Cheers
Ian