Master Cylinder issue???

I just had the same MC failure a week ago, on the last leg of my trip back from Arizona to Washington in my 69 Barracuda. The brakes behaved pretty much as you described, but no sign of leakage from any wheel cylinders. I did note the large reservoir fluid was down slightly. As it turned out, upon removal, there was a slight leak from the back of the MC, but not enough to cause the loss of breaking. The shop said the MC seals had failed and the fluid was bypassing the piston when I stepped on the brake pedal. If you want to maintain the original look, then purchase a new premium quality MC that's correct for your year/make car and brake setup. I've bought a few things from Rock, but not the replacement MC. I would shy away from a rebuilt MC in any case.