Yeah, that missing spring makes me wonder. Ford's pressure differential valves are designed to float, but they don't have springs on either side. A spring on only one side doesn't make sense. I know the repros include a spring.

I can think of three possibilities:

• There never was a spring.
  
• There was a spring, but it has completely rusted away.
  
• All three of the valves I've disassembled were opened before and the spring was lost.
  

I'd like to believe that maybe the spring rusted away, but even then I'd expect to find some small rusty remains.