Brake issues - lost brakes, but no leaks found?

[InNeedofaDelorean]

So I recently bought a 1967 GMC 3/4 2wd, and I lost brakes on my way to work. It's in the shop, and they refilled the brake fluid and found no leaks - none of the lines are leaking, and we're all scratching our heads as to where it leaked from, or where it went. I know there was a bunch of fluid dripping from the transmission pan, and from other parts near that area, so there is a leak somewhere, or something burst at 60mph and isn't doing it right now. Any ideas? I'd like to be able to drive it without dying, haha.