OK all – I just had to do it … I cut open and hacked on an old BBC fuel pump. It appears the lower “can/reservoir” on the long pump is full of fuel. There are two check valves and the “bypass” is nothing more than a tube with a very small hole (maybe .010) in the tip. From right to left are the components – lower can w/fittings, check valve plate, diaphragm, spring, seal assembly and body.
I am sure if I had a “short” pump to hack apart – it would be pretty much the same except for the large fuel reservoir on the bottom.
Not drawing any conclusions, but it appears GM attempted to provide more fuel within the pump itself … TAZ
