66 Power Drum Brakes MC questions

[buildinga55]

We split the lines and went with a booster and dual reservour on the fatherin laws 66 about a year ago. I thought when we took it back to him the brakes were decent, but not great. I was in it today and the brakes are not stopping the truck well. The pedal is going down about half way and then the truck coasts to a stop. He says at times it has better brakes than other. It has a mild cam in the truck and I am going to route the vacuum line to the intake instead of the carburator and see if that helps. I am wondering if the Master cylinder I choose is not the best for this application. At the time I know i researched the correct MC, but does anyone have an Oreilly part number on what they have used with success. This is a drum drum setup. Thanks