Really strange brake issue..

[AllenKarber]

Or maybe not...

I've been working on getting my '64 running again. It sat for a year,and when I pressed the brake pedal the other day it went all the way to the floor. The brakes were fine when I drove the truck last. While I was sitting in the truck and pumping the brake pedal I kept hearing a whooshing noise. I finally figured out it was coming from the gas tank filler neck. How in the world is that happening? Am I missing something?? I have manual disc/drum brakes with a dual reservoir m/c.