I inherited an old pressure cooker from a friend, but it seems to be missing a part. I locked it down and boiled some water in it. 
So it looks like they had some tape or something over the red arrow valve. The green arrow valve never popped up, and I was unable to get any psi on the gauge. Once the water came to a boil, a vent of steam (and a bit of water) came out of the red arrow valve.
Can anyone tell me the name of the part that is needed at the red arrow?
Thanks.
I don't have the reputation to comment on your answer. Thanks for the help. The only indication of model or type is "LISTED 409A" on the bottom of the vessel. I'll be sure to get the proper pressure regulator.
Second edit. Actually found the model number. 0178107. 26091G3
Best Answer
It's good that the "green arrow valve" never popped up; that's the safety fuse, and only "pops" (destructively, and probably spraying food onto the ceiling) when the cooker pressure goes way too high.
The missing part is the pressure regulator, gauge or counterweight. The pressure cooker won't work without it. It blocks steam coming out of the vent until the steam pressure gets high enough to push the regulator up off the vent, thus limiting the internal pressure.
These are usually pretty easy to replace, as they are easy to lose.
It's important to get the right one, to use the wrong one could be very dangerous!
Your cooker is made by Presto. Presto calls that part a pressure regulator, as well as I can tell: http://www.pressurecooker-outlet.com/Presto-Pressure-Cooker-Parts.htm