Rear Master Cylinder Push-rod Pin
This can be a bitch to get out. Try depressing the brake pedal as far as it will go. This positions the pin into a spot where there is a little more room. The lower you can press the pedal the better. The pin will hit the exhaust collector just before it clears the spline from the brake pedal. You may be able to remove it by pushing the pin up towards the top of the collector and working it out. An easier method is to loosen the front nut of the two nuts by the push rod bracket that accepts the brake pedal pin and unscrewing the rod from the bracket.