Hey guys, just wondering if someone can give some insight with my brakes.

Info:
1971 1200 Ute
Dual master cylinder(originally single master)
120y discs up front
standard diff
new pads,and shoes

Problem:Occasionally when I was driving(on single master) I would engage in brakes and the pedal would hit the floor without any stopping power. After a few pumps the pedal comes back to life and I have brakes again...VWHA-LA!...Anyway, what I got told by a local mechanic that disc brakes use more fluid than drums,so I needed to upgrade my master...I upgrade the master to dual system allowing the front and back to have their own resiviors, I salvaged this master off a ute I previously owned(was working fine in that ute). Same problem still occurs...After close inspection I see my left wheel cylinder has a leak. I then replaced both left and right cylinders...I bled the brakes then take it for a spin and the problem still occurs...They hit the floor with no power and then after a few pumps the come back to life. I have no idea why this random loss of brakes keeps happening. Any advice would be highly appreciated, cheers! **Forgot to add, a mechanic said that my drum assembly on the rear has the trailing/lead shoes the wrong way around...