im in need of replacing my brake master cylinder because the old one is rusty inside. Im going to use the drum brakes and was wondering if the master cylinder for the drub brakes is the same as the one for disc? can i use the master for disc with the drum brakes and still be safe? i plan on upgrading to at least power drum brakes or even power disc at a later date. but for now is there anything i need to keep in mind when looking for a replacement and still be safe?  

The master cylinder for drum brakes is not the same unit as the disc. After 66 Ford realized that disc brakes need higher pressures so power units were standard on all disc brakes. I advise you to match your components, whenever you say you have this now but will change to that in the future,,,its a compromise and not the best thing to do. Bite the bullet and buy a matched set of disc brakes and power master cylinder from a reputable company. Then the pressure and volume(generated by the master cylinder) will be correct and your brakes will work properly.
Lots of guys mix and match, some of them get lucky and some of them have problems and dont know why.  

Thanks for the info, thats something im not familiar with but now im understanding how things work. i have been looking for the original dual resivoir master for the manual drum brakes because that is what im putting back on it for the time being. later on once im driving it a while and doing the usual upgrades and tinkering ill probably swith over to at least power drum brakes. my original thought was would the power drum master work with the manaul drum brakes or would it over power the brake cylinders and blow the seals.  

You can buy an original dual MC from Autozone for your 68 Mustang.  

The disc brake caliper has less movement than a wheel cylider (travel). The master cylider and the proportioning valve will be different because the caliper doesn't need to travel as much or move as much fluid and as mensioned the pressure is different. The power brake booster is more of an "assist" to make pedal pressure less or the feel of resistance on your foot less. It won't creat greater pressure at the wheel cylinder as that is controlled by the master cylinder piston as mentioned before. You should be able to add a booster but the pedal is differnt as well. The fulcrum point is differnt on a power brake pedal so you would need the booster and pedal (the whole thing, not just the part your foot touches) for a 67/68 Mustang.  

Translated. Tarafield1 is saying that if you have disk brakes on the front and drum brakes on the back, then you need to have the correct Master cylinder for disk brakes.  I have the dual master cylinder for drum and shoe brakes both front and rear on my 65.  My brakes work just fine.  But, the 68 is a little heavier, so you might want to consider the disks with the power booster if you can afford it.  Otherwise, just go to any parts store and get/order a new dual bowl master cylinder and put it in after you have bench bled it.