i presume your bleeding the fuel system.
the cap can be on, for the amount of fuel your trying to get through the pipes, and at the speed, the cap on or off it wont make a difference.
dont block the return pipe when bleeding as this will make things slightly worse, (in theory).
is it petty or deisel?
if its diesel, bleed to the pump via the bleed screw on the side of the FI pump, then tighten that and crack off two or three of the injector pipes at the injector end, when clean fuel comes out , not foam. then tighten them up and off you go.
if its petrol, just keep turning her over, untill ther float chambers get some fuel in the and it starts spluttering and then fires up.
