Brake Master Cylinder Help

If you had manual brakes, then you must have had all drum brakes to begin with. Your post implies that you now have disk brakes on the rear. If thats so, what is on the front? If you have drums on the front and discs on the rear, you should change the fronts to disc as well. At the very least, you will have to have a proportioning valve on the front brakes, not the rear.