Well, first, try the optimistic path. Find the master cylinder, re-fill it with fresh brake fluid, and try pumping the brakes then. (This is the "someone must have just stolen the fluid out of the master cylinder" theory of brake repair.)
This doesn't solve, or even identify, the problem, BUT, it may get you back on the road, at least temporarily. Brakes will want to be bled, and I'd take it slow, easy, and careful while you test the system. Your parking brake works, right?!
I would guess a wheel brake cylinder has crapped out. Evidence might be found by inspecting the insides of each wheel for signs of fluid leakage. Or, yeah, the hydrovac might have gone wonkey.
-Michael
