The return spring is to allow fluid into and out of the master cylinder as the thrust bearing/ clutch plate wears.
Is the hydraulic pressure insufficient to open the check valve in the master?
hydraulic pressure in the master cylinder/ system shuts the valve to the reservoir, the weight of the pedal is enough to create pressure. If hydraulic pressure opened the valve to the reservoir the fluid would go there instead of moving the clutch. Thats what I thought the spring was for, I’ve been wrong before though. 
When I crawled under the dash and looked at how the shaft off of the pedal contacted the master without the spring I didn’t see any discernible movement of the master just from the weight of the pedal. It may vary car to car or he movement might not be enough to visually detect. Regardless, I installed the bungee (a spring would work as well) because the Jaguar engineers who designed the system apparently thought it was necessary to keep the weight of the pedal off of the master. That was good enough for me.
Yeah, the pedal wont move the master cylinder piston, But the fluid in the reservoir only gets into it under its own weight, so if the weight of the pedal is more than the weight of the fluid none will be able to pass the ‘check’ valve. Also Jaguar put it there 