ABS problem

Alright first off all, what is that hydraulic unit under the ABS modulator (95 Camaro Z28)? there is a horrible leak, and i hardly have any ABS infact the car will just roll to a stop even when slamming on the brakes. i keep the master cylinder filled with brake fluid, but it has a pretty good leak rate. I just need to know what the part is called so i can replace it. it has brake lines going into it. I actually bought a new modulator, but its a part that actually bolts on under the modulator. i havent found anybdoy who knows what it is, so any help would greatly be appreciated