I have the AZ Zcar brakes and I'm still using the stock '78Z master cylinder. My brakes are fine.
When I first installed them I had problems similar to what you are describing - soft pedal, seemed like you had to stand on them to get the car to stop. Pretty much couldn't lock the wheels under any circumstances.
Several years later I decided to try a different set of pads (carbon kevlar) - this time I properly bedded them in, and these pads made a world of difference - firm pedal, and I now had to be careful not to lock the wheels on hard decel.
I've since changed over to Wilwood's 'Gator' pads and they work fine, too. I don't know if it's the pads, or the fact that I properly bedded the new pads in, but they work really well now.
BTW, you won't be able to get away with not using the prop valve for long once you get things working properly - I have my rears backed all the way out, and it's just barely enough. One time a couple of years ago after working on the brakes I forgot that I had the prop valve misadjusted and put the car completely sideways when I tried to stop late for a red light. Had to sit at the front of the line with the car sideways in the lane until the light turned green. :shock: