If you swapped from the small drum front brakes to the disc brakes, the master cylinder is not the same. As pads erode, the disc calipers require much more fluid during actuation than the small wheel cylinders you replaced, which means when the drum MC is at low fill (pads wearing out), you run the risk of accidentally purging the MC. I suspect the only reason some of my friends get away with it is because they never let the fluid run down enough to "test that theory"...
While I have heard of guys running the drum master with the discs, I've always changed the master to ensure the larger reservoir matched the capacity of the caliper pistons throughout the life-cycle of the pads.
The best way to approach it is to look at the master cylinder from the car you pulled your brakes from. Chances are it will have the larger front reservoir, which should be matched to your set up and, as noted above, get the proportioning valve if possible, since you will be doing some re-plumbing anyway.
Just a suggestion and good luck on that hotrod !
Southernman
