Brakes are a sealed system. You should NEVER need to add fluid unless you're bleeding the brakes or there is a leak.
First, check your pads. If they need replaced, your fluid will be low because it will be in the caliper rather than the reservoir.
Second, check for leaks. Check at the master cylinder first, both inside the cabin and at the booster, then work your way through the system.
If it was low enough you got air in the system, you should feel it in the pedal.
ABS in these cars really sucks. I've had 2 91 VR4s that it worked fine in, which is somewhat rare, actually. I kept it because it worked, and i never encountered any problems with it. It actually saved my butt a couple of times. I've had others where it never did work and i couldn't figure out why, even with the MUT. I suspect it was the ABS unit. Most of us either ditch that unit completely or disable ABS somehow.