That "disk" is part of the function control switch to determine vacuum path to operate the vacuum pods in each mode. If you find a new one, you will likely pay out the nose for it, unfortunately. Used to be a $9 part. Best price I've seen lately is $75 plus shipping. GM p/n 16037778.
It COULD just be a problem with the dual vacuum actuator pod with the yellow/red lines going to it. It controls the lower door which fortunately, isn't that tough to reach. GM p/n 1996729 if you need one. They can be found on other G-bodies and possibly earlier 70's A-body cars. According to the parts information, older part numbers were 1996574 and 1996628. An interesting thing is that the older parts BOOKS show 1996564 was used on the A-body cars. Which has a different actuator arm than the later G-body. So if it's not a 1996729, not sure if it would have the right connector on the business end. It's going to be a PITA no matter what the issue is here.
This may give a bit of insight.
They call it the lower mode valve. See the locations here.