Oh woops, yea, it's on my 78 280z. 
  
It's right next to the water control valve, but what's weird is that the water control valve doesn't full shut off coolant flow, while the other one might (the vacuum capsule i mean, and mine's pitted and leaking). 
  
It's like if water supply is shut off completely by that the vacuum one or something... 
  
Mario