M/C for drum/drum, to disk/drum?

[stealthy '69]

I'm installing 71-72 disks on the front of my 69, and I'm wondering if I NEED to change my M/C or if I can just add a disc/drum prop. valve. I'm trying to do this on a pretty tight budget since my wife doesn't have a job. Any help would be great. Thanks.

[mclairmo]

Yes, you must change to a disc/drum master cylinder because of two reasons. First, the drum/drum master may have a residual valve built into it that will cause the discs to drag. Second, the disc brake calipers require more fluid volume to actuate their pistons than do the much smaller drum brake wheel cylinders. As such, using a drum/drum master for your front discs could cause the fluid to run too low. If you look at a disc/drum master, the seciton of the resevoir for the discs is always larger than the one for the rear drums. You also need to run the disc/drum combination valve (erroneously called a prop valve by most people). A combination valve will include a metering valve for the front discs, a residual valve for the rear drums and a proportioning valve.

[stealthy '69]

Thanks for the expailation. Now I know I need to save up a few more bones for this deal.