Here it is.

BTW Note the weird pressure gage. It goes from
30 inches Hg vac to 30 inches of pressure above
atmospheric. Good kind of gage to use in your
cooling system to be sure the system is functioning
properly.

When the engine cools off the water jacket goes to
negative pressure and sucks the coolant back in from
the over flow tank if your system is working
right and there are no leaks.

I learned this trick from Ken Welter.