Vanishing Brake fluid? Mystery solved.
 

So last week I get in the car to got to work and the brake warning light is on after releasing the parking brake. Reservoir was low as I expected though odd since it not below freezing( grommets under the reservoir leak when it hits about 25F or so). Filled it up and went to work. Checked that evening before leaving work and was still full. Check the next morning empty but can't see any visable fluid any where. Fill up go to work. Same thing full in the evening empty the next morning. At this point I just start driving the truck and order a M.C. from Rockauto. So today I take the M.C. off. I also remove the booster because it is rusting from the leaking grommets and I am putting my spare from the parts car on it. Lo and behold it sounds like its got liquid in it. Mystery solved. My brake fluid is in the booster. I guess it's got about three reservoirs worth in it.

Check the firewall inside the car under the carpet.

Seems to be all dry.

^Good that you figured it out. If the fluid ended up in the booster, I'd probably get a new booster just to be 100% sure its internal rubber/plastic diaphragm parts weren't chemically compromised by the brake fluid exposure since you can't clean/inspect those. I'd also check the condition of the booster's vacuum hose & check valve, and either replace those or clean/inspect them to ensure they are still good. When the engine is running, I wouldn't be surprised if a fair amount of that brake fluid got sucked into the engine under vacuum & burned.

Luckily I got a spare that I hope is still good. I'll check those other things while it's apart. I have some painting to do on the frame rail while I can get to it with the booster out.