The pressure is maintained in the system by the constant flow from the pump to the restriction that is the pressure regulating valve in the fuel filter. If there is a one way valve in the pump assembly it is a particularly rubbish one but some pressure does stay in the system for quite a while after the pump has shut off and the initial drop from operating pressure. Can't say I feel like stripping the assembly down far enough to see if there is something hidden in the plastic...and even with the dedicated non-return valve I fitted between the output of the pump and the engine on my one, once the pump is off the pressure falls away fairly quickly, and yes, I do have a good fuel pressure gauge on my dashboard.
