[Zoomad75]

It can leak into the booster. But, I'm doubting it since your loosing fluid through the front reservoir. Heck I had my non-power assisted Nova leak at the master cylinder before and couldn't find the leak under the hood either. That was until I looked under the dash and found it had been dribbling inside for some time. Master cylinders can leak through the seal where the pushrod moves it.

I just can't figure out how the front could leak past the rear and still not have any leaking from the rear.

I just did the master cylinder on my 75 and I can say mine was leaking from the rear of the cylinder, but it was leaking out from between the cylinder and the booster. Leaving pretty clear evidence of the problem and where the fluid was going.

It's not that hard to remove and you can at least pull it off the booster to look for a leak without cracking the system open and removing the lines. There's enough movement to slide the cylinder off the studs to see if it's been leaking or not.