If the master cyl. is made for drum brakes it should have check valves built in. All the old single master cyls. had a check valve built in the rear of the cyl. Make shure your shoes are adjusted up close to the drum. If that,s O.K. you still have air in the system. The master cyl. push rod must come all the way back so that port opens up. You can tell which part of the system is causing the problem by pumping up the brakes & releaseing them. Pick the lid up on the master cyl. & a stream of fluid will be shooting up on the side that is causing the problem.
