You could have a dead spot in the master cylinder. If the fluid is old it might be contaminated with water.
Best way to describe the problem hmm....there is a type of piston in the master cylinder. It constantly moves in one small area, creating a smooth surface. If the fluid has moisture in it, a rust ring will form in sections on that smooth surface and the piston can't move smoothly over it. If you slam the brakes, the piston will move quickly and power its way through that ring of rust. Going slow its not easy for it to make it over.

 

the fluid is low though

 

did you fill it back up yet?

 

So what is locking up? Your rear or front brakes? I would bet you need to rebuild the wheel cylinders if the rear is the problem. Not an expensive ordeal to do.

If it's the front and they are locking up I would bet it is your calipers and the pistons need to be removed, the cylinder honed by hand with a 500 grit sand paper or honing stones if you have them, then cleaned, reassembled and that would end of the locking up.