How do I isolate the leak?

Thanks for the tips guys. A lot of useful help. To answer you question yes I did check the drums. I said in an earlier post that I adjusted the drums and replaced the wheel cylinders already. I inspected them and everything looks good. Good shoes good springs nothing rusted except for the wheel cylinders that I replaced on both sides just for the hell of it. I will check for air in the lines today as I am off. Also I will do the backup and brake experiment to see if it improves the drum braking ability. Thank again!