HouseCallAuto 2/15/2014I would guess that the fluid height sensor within the master cylinder reservoir is sticking and giving a false reading. The abs light turning on at the same time might be a result of the low brake fluid indicator, in other words it may disable the abs (turns on the abs light) if the abs module is being told its low on fluid, even if it is a false reading. Try excercising the float with a suitable tool / pencil or whatever works.
HouseCallAuto 2/16/2014I misunderstood. I missed where you said that the fluid was actually disappearing and needed to be replenished. OK, you have a leak plain and simple. Brake fluid does not evaporate. If I were to look it over I am certain I would find a leak, either from a rusted porous steel line or a caliper piston seal or whatever. There is a leak. It can be diagnosed by someone that knows where to look.