You are talking about a residual valve. If the car has disc on the front and drums on the rear, you use a 2 lb valve for the front line. If it has drums all around you use a 10lb valve.
I have plumbed the 2lb valve in my lines right into the lines coming from the master cyl.
These residual valves help keep brake fluid from draining back to the master so the pedal stays firm.