308 GT4 Brake Pedal Assembly Failure

That's not a bolt, it's a pin. A cotter pin goes in the small hole. Number 38 on the diagram. Just take your pin to a hardware store and get a cotter pin to fit.
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I think you might find it easier to access it through the removable plate on the top of the pedal assembly than sitting on your head and dropping a cotter pin in your eye.

 

Just very recent, I too reassembled my peddle assembly and wasn't able to feed this pin through the top as the diagram is showing, so I installed it upside down and add a thick cotter-pin and bent the tails around both sides of the pin to keep it form coming back out during brake operation... I still had some concerns with this to be honest. The pin slightly to tall to go through the access inspection plate as Brian stated.

 

It could also be #36 since it fell on the floor mat

 

Just to be clear, #36 is the pivot pin in the OP's photo, #38 is the cotter pin that he was missing.
We haven't heard back if he fixed his problem.

 

There are actually three #36 clevis pins in that SPC diagram -- the OP didn't say, but it must be the one that connects item #31 to the brake pedal arm #9 that fell out (as the other two #36 have gravity working in their favor to keep them in place even with no cotter pin present).

 

Such an unlikely failure. I’d give a close look at the others

 

#66 (part #16044321, "vite fiss.") is a screw that holds the large casting to the car body, right rear position. "vite" is "screw" in Italian, "fiss." seems to relate to secured, stabilized, permanent depending on what "fiss." was an abbreviation for.

#68, also a vite, holds the left rear corner and #42 (2 required), "bullone" (bolt) the two forward corners of the casting. Then again the diagram may be valid only on the day that it was drawn. A week later the casting may have been held down with 4 pop rivets and some bubble gum. Life with an Italian car can be quite challenging at times.

 

I realize that you are probably already tired of messing with this, but did you try removing the other Clevis Pin to allow moving the linkage as described in this jpeg to get access to install the Clevis Pin that fell out in the correct (safer) direction?:

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If you do stay with the Clevis Pin installed upside down, I'd recommend:

1. Use a flat washer between the Cotter Pin and the Arm surface that it is presently contacting (the Clevis Pin should be long enough to accommodate this), and

2. Use a Cotter Pin with a larger diameter that more closely matches the hole size in the Clevis Pin.