rubber high pressure fuel line diameter?

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I'm trying to troubleshoot a small leak somewhere in the fuel line between the sending unit and fuel filter and need to know the exact fuel line diameter for the high pressure rubber line - is it 1/4quot; or 5/16quot;.

Do you see it spraying or dripping?

I see dripping after the car has been run and its intermittant and very light.  throughout the day, I'll get a small drip from underneath the tank near the fuel filter.  I'll park the car on the drive for a few hours, than put away in the garage for the night and there is no dripping on the floor overnight.
filter and hose connections to the filter all ok, top of tank is all dry and ok but there is fuel dampness across the front of the tank and underneath.  Where the front of the tank is shallow, I can feel puddling on the flat section on top.  The leak is not dependant on fuel level in the tank so I'm ruling out a leaking tank.  My best guess is that the steel line has a small split and is spraying a small amount that pools on that flat spot and than drips slowly over the edge.  I''m hoping a $10 peice of hose helps be test my theory before tackling the tank removal.

I think it's 8mm, therefore 5/16quot; will fit.

5/16quot;

thx for the quick replys.  My hunches were correct on the source of the problem.
Another question.  On my tank, to the left of the fuel sending unit plate (looking towards the back of the car), there is another small plate with a hose and what looks to be a one way check valve leading away from the tank.  My Hayes manual mentions nothing of this separate hose.  When I was disconnecting and reconnecting the fuel pump electrical clip, the  check valve would buzz/gurgle for about 20 seconds each time.  Any clarification on what this is?