extricate,
The FCOM description is very close to the money without going into too much detail. Here's a little bit more for you.
Fuel tank venting is accomplished with sealed hat-shaped stringers attached to the upper wing skin inside the tanks. These stringers are interconnected with drain and vent tubes and terminate at the surge tank.
The stringers vent via the surge tank to atmosphere through a flame arrestor and ram air NACA scoop on the lower surface of the wing near the wingtips. The NACA scoop provides a positive pressure head through the vent system to the tanks during flight.
Pressure relief valves (can't remember if there's one or two) located near the NACA scoops on the under surface of the wing protect against tank damage due to excessive positive or negative pressure if the flame arrestor becomes blocked. The valves are operated by either fuel or vapour pressure and open at a positive pressure of 4.0psi or a negative pressure of 1.25psi. The valve poppet will move upwards into the surge tank skin surface (approx 1 inch) therefore indicating a differential pressure exists either positive or negative due to a blocked vent system (more than likely the flame arrestor) Hope this helps.