vapour lock

His hand gripped mine like a vapour lock.

When the engine is cold and there is no vapour lock in the pipe the pump will deliver the fuel more easily.

Early production Spitfire IXs suffered from vapour locks in the fuel lines resulting from fuel evaporating if the aircraft was parked in direct sunlight.

Gascolators are susceptible to the effects of heat from the co-located engine and exhaust components, which can cause fuel vapourization in the gascolator and vapour lock problems.

Either you have a faulty fuel pump which allows fuel to bleed back in to the tank or the petrol pipe is routed too close to sources of heat which is causing vapour locking in the pipe I would guess it is faulty.

The fuel system incorporated a relief valve that returned fuel to the tank when pressure in the leads to the carburetters exceeded 1.5 psi in order to reduce the risk of vapour locks occurring at the engine's high operating temperature, while the car's battery, unusually, benefited from its own thermostatically controlled cooling fan.