4 psi sounds about right, anymore than that will cause the carburettor to flood. If it is holding that pressure, then it tends to discount the diaphragm being holed.

You say it has 3 fuel tanks, so the symptoms suggest an air leak which is common to all three - try looking down-stream of the tanks, such as unions past where the fuel lines become one; or pinholes in the pipe. The fuel volume at cranking speed sounds OK; I thought you meant with the engine idling; which tends again to point to an air leak somewhere.

Check out the pump thoroughly; a new diaphragm and gasket wouldn't cost much; and no it is not normal to have bubbles. Change all of the hoses for new ones,

Cheers Charlie