Moisture in Distributor Cap???

[4 SPEED]

Started tuning up my 74 today and when I removed the distributor cap to my suprise there was all kinds of corrosion on the terminals on the inside of the cap.
In fact all the brass turned green probably from moisture.
The rotor was also messed up and the distributor was rusty.
No other part of my car has any rust so how did the moisture get inside the cap?
The cap is five years old but only has 7k miles on it.
The Vet is garaged.

 

[GT6Steve]

As the cap heats and cools the air in it expands and contracts. The cooling contractions draw in humid air which then condenses inside the surface.

For years I have fought "watertight" electrical boxes which suffer from this in humid environments. In the mine I frequently found the very best sealed boxes filling up with water because the conduits leading to it weren't sealed and breathing the humid air in and out of the box.:smash:

 

[SuperBuickGuy]

in 4x4 applications (which is my first love) we run a heat riser to the distributor cap (external) with a hole drilled in the top to allow the moisture to escape. On a 71 Landcruiser with a Ford 302 it was the only way to keep it reliably running. Alternatively speaking, a block heater could help you out because it'd keep the engine compartment warm (50*) and should mostly eliminate moisture.....