[screamingchief]

With a mechanical pump and "smallish" lines on the suction side look for ANY restrictions that are adjacent to ANY heat source.

Restrictions cause a pressure drop (vacuum = extremely low pressure) this low pressure reduces (lowers) the boiling point of the fuel in the suction side of a fuel system,same deal as coolant in a cooling system,pressure raises the boiling point,so rarely is the problem on the pressure side.

Fuel already has an extremely low boiling point (it can start "boiling" to a small degree at room temperature & amospheric pressures,this is why it evaporates easily),and putting it in a system with a vacuum that lowers the pressure just makes the situation worse,add some unwanted heat load (heat increases the chance of boiling,,,right...) and pretty soon the fuel is boiling and the suction is unable to overcome the boiling fuels positive pressure lock in the line.

On mechanical pump fuel systems it's often the use of the forged 90 degree elbows on the inlet (suction) side that create serious restrictions,that or a restrictive filter(s) on the suction side.

Give the system between the tank and the pump a real close exam with an eye open for anything that resembles a restriction,especially if it's close to a heat source.

I myself would have used -8AN for the lines between the tank and the pump,and absolutely no "forged" 90 degree fittings that are'nt the large radius/gradual "bent tube" style elbows,filter for the suction side needs to be 80-100 micro "zero pressure drop/high flow" style filters with a GPH rating to match the pump,JMO.