BE CAREFUL
Most front-drive cars have the brakes diagonally connected - left front with right rear, and right front with left rear. The purpose is that if one circuit fails, you have about 50% braking left in either case. Otherwise, if the front circuit failed and you only had rears, you are really in trouble. Rear brakes on a front-drive are deliberately designed to not do much.
Here's the potential problem; in the old days, there was always a proportioning valve in the circuit to the rear that compensated for the rear of the car unweighting during hard braking (it limited the brake line pressure to the rear once the pressure got to a certain amount). Because of the diagonal connection, front-drivers would have two such valves, one for each side. But when ABS came to be, in most cases, the proportioning valve was eliminated and the ABS is used to compensate for its absence. Disabling ABS might cause some unexpected side effects over and above the obvious loss of ABS function alone. There's no way to know this without talking to someone who has done it on the same vehicle. (The Germans call it EBD = "electronic brake-force distribution")
I know that my own car (which is not a Mercury Sable a.k.a. Ford Taurus!) disabling the ABS results in premature front-wheel lockup to the extent that the car becomes a real handful on slippery surfaces. It seems like in this case, you are applying the front brakes and the computer is telling the rear brakes what to do (and that is how the EBD works), and it defaults to front-brakes-only if the ABS fails. They do not want you driving around with the ABS disabled.
