Using a disc brake master cylinder in a drume brake car.

The purpose of a residual check valve is to retain a small amount of line pressure, about 2-3lbs. Without it, pedal travel is increased to make up the difference. Disc brakes use no such valve as that would not allow the caliper piston to retract. Often this "residual check valve " is a small rubber "flapper valve" under the line fitting in the M/C.