One of those slow leak jobs sounds like. MY 47 was doing the same thing. It would hold brake fluid for about 3 days but after a week the master was empty. Once you locate that master cylinder (it should be under the cab floor attached to the frame as mentioned above) refill it and pump till you get the breaks back (may have to bleed everything but may not too.) Look at all the drums see if the bottom is wet any where (if so you got a slightly leaking wheel cylinder) If not trace your lines check for any lost break fluid. Lines on this unit are simple. YOu have a single line runing from the master cylinder back the left hand frame rail. At the back of the cab there should be a support the break line crosses to the right hand frame rail on this support. At the right frame side of this support their should be a cylindrical thing the line runs into. This is you hydro-vac break booster. (I'll come back to this later)The line then leaves there and splits to the front and the rear running on the right frame rail.
IF you can't find any leaks you probably have a bigger problem...
Let the truck set for a few more days and see if you loose your breaks again. If so your hydro-vac is probably leaking internally. Mine was. I'm currently looking for a rebuild kit but for now I by passed it. Still stops but have to really put some foot into it.
I got by for two years adding fluid every time I drove it cause I didn't want to take apart the hydro-vac unless I knew it was it. Finally got my proof when brake fluid was dripping out of the vacuum fitting on the truck.
Good luck!

1947farmer

1947 2-Ton Chevy
Story in the Stovebolt Gallery
More pix in Photobucket
1977 3/4-ton
1985 2.5-ton
2- 1947 Allis WC
1- 1949 Allis allcrop
1- 1955 Allis WD-45
1- 1976 Allis 7040

"If it ain't broke you ain't trying"