All drum brake master cylinders have residual valves, usually around 10 lbs, in them somewhere, usually behind the brass seats that the brake lines hold in As already mentioned check that now by removing them with a # 8 sheet metal screw turn in 2 or 3 turns and then pry the seat out, the valves are rubber flapper valves with a small stainless spring inside the valves Remove them and throw them away and resaasembe the seats , make sure and crank down on the brake lines but don't strip the threads out of the master cylinders Same thing on the new matser cylinder also. On cars that have the master cylinders lower than the brake calipers you will need a 3 lb residual valves (Wilwood sells them )so the brake fluid won't drain back and overfill the master cylinders and leave air in the brake calipers