Sounds like you need a vapour barrier between your interior skin and the frame/insulation - bit like a campervan. You'll also need some sort of ventilation then, like breather bricks in modern build houses, or a roof vent to let out the hot damp air.
I could be interested in a pair of a brake levers if you split?
Now I'm no expert but are the mechs arabesque or later? This era road stuff is new to me, so I'm up for some education.