I think the full stroke on a '39 Master Cyl is one inch. I should think that the brakes should be full on (panic stop) at no more then 2/3 (pedal) stroke. If you are dealing with new brakes, you may have to fully get them beded in before you can get the proper adjustment.
What is your application? If you are having trouble getting proper adjustment and you are dealing with a non-stock application and not a stock '39-'48 Ford or Merc., you may have to re-visit your installation. There is a lot involved with proper lever lingths and linkage with a "custom" brake installation.
Let us know what you are dealing with.
Chris.