Look under the brake booster and the vacuum reservoir. I have been cleaning surface rust here this afternoon, revealed after removing my leaking master cylinder and the booster for good measure.
There is some white semiplastic seamsealer, used in various places, and in this area lt all had some surface rust underneath. I dug it all out and have hand-sanded the surface rust away. Fortunately there is very minimal metal loss; I will treat, then prime and paint this area, and should be good to go, especially if I can prevent brake fluid from getting everywhere.
I post a couple of photos of the area. In the left photo, the dark area on the left is the opening through which the brake pedal descends to the footwell. this is what one sees after removing the cast aluminum bracket which supports the brake pedal and brake booster. In the photo on right, one can discern the horizontal shelf underneath the vacuum reservoir. The electrical harness plug in on the firewall, just outside the brake booster.
Also one can see in the bottom-center of the left photo, a cable which comes from the accelerator pedal assembly to the cruise control bellows. It seems to have a small white rubber grommet which fits in a hole in the horizontal firewall shelf; the only thing securing it in place seems to be a glop of thick dark sealer. After painting I will have to replace it somehow. Somewhat cheesy way to attach it, seems like.