If these are the small black hoses attached to the fuel filter and looping from injector to injector ending with a short plugged hose then “return lines” is a misnomer. These lines carry filter fuel under pressure to the upper portion of the injectors. This fuel apparently acts as a lubricant to the upper workings of the injectors. Fuel is under pressure coming from the fuel filter while the engine is running. As there is a least a check valve preventing the reverse flow of fuel back through the fuel pump perhaps pressure may be retained after engine shut down. there most likely is a check valve in the return fuel circuit to the tank that would possibly have a high opening pressure to ensure that fuel is directed into the IP an only allowing return to the take at a relatively high pressure. There is a return line on the engine side of the IP that returns excess fuel back to a banjo fitting on the fuel filter on my 603 engine.

Re: the sucking sound at the fuel tank cap could be the result of an obstructed fuel return line, maybe clogged maybe a faulty in-line check valve or a faulty tank breather. Fuel is sucked from the tank under vacuum but is not returning under pressure hence a vacuum is created within the fuel tank! Theoretically a condition of fuel starvation could exist either at low ROMs reduced suction and return pressure or at high rams with greater suction and increased fuel tank vacuum.