John, that brake fluid had to go somewhere? It does not evaporate! I would fill it with the proper fluid, and bleed as needed, but check for wet marks along the frame, at all the junction points, and at the bottom of the brake drums on the inside, and the inside of the tires! You will find the leak, as it will show itself, as it gets to where it leaks more, as time goes by! I would suggest you keep a very close eye on the brake fluid level, till you locate the leak! My guess is a wheel cylinder inside the brake drum, if you have them on the rear, if you have disc's all the way around, the rubber hoses that go from the hard lines to the calipers would be the first place I would look! The brake fluid has a distinct smell, and your nose will help you find the leak also! Good luck! Rail!
Retired, and "Always on Holiday!"
1996 Monaco Windsor 38PB, "Mona" 275 HP., 8.3 Cummins, 3060 Allison 6 speed