The master cylinders that are used for drum brakes use a "residual pressure check valve" in the drum brake circuit[s]. If you are using disc brakes on this type of cylinder, you will need to disable or remove this valve, otherwise, the small amount of residual pressure causes the brakes to drag, but then you knew that.

I tremble when i think that there are people out there who are messing with brake systems without having a FULL UNDERSTANDING of ALL the principals of operation. The potential consequences of this are too frightening to contemplate.
Chris