Brake fluids are glycol ethers. A glycols are HC chains with an OH group at either end (and are sometimes called di-alcohols); ethers are HC chains with an oxygen in the middle of the chain. A glycol ether would have both.
Glycols are hygroscopic, normally a downside, but actually a plus as a brake fluid. It prevents the formation of a two-phase liquid where the water phase could freeze. Their other main advantages are: they do not degrade rubber parts (found all through the brake system) like mineral oil and they have an astoundlingly high VI, so viscosity remains fairly stable through the entire operating range.
Is there data to support that the insoluble brown portion is glycol ether & water forming a crystalline matrix? I always figured that was iron oxide & other particulate matter. Did you try filtration before vacuum dehydration?
Oh, there's no way you'll get 29"Hg from your car, or even most lab vacuum pumps. They top out at about 26"Hg. To get higher than that, you'd need a cascade system.