1973 Dart brake failiure

No. That's the reservoir for the front brakes. Look for leaks on the bottom of the back sides of the backing plates and also inspect the brake hoses to see if one or the other or both are ruptured. Also inspect the hard lines for rust through as it does happen. If you see leaking from the back sides at the bottom of the backing plates, the front wheel cylinder(s) have probably failed. They should be repaired or replaced in pairs.