[FC] Brake specialists?

[corvanatics@corvair.org]

I've been working on a rampside all summer. It's been on blocks since May. 
When I first got it, you had to pump the brakes several times to get them to 
work. I bled the brakes, put in all new fluid, and they worked fine (on the 
blocks). I checked them every so often, and they were still holding. I had not 
checked them in about a month, and when I checked them yesterday the pedal went 
straight to the floor with no resistance. I can't see any fluid leaks anywhere. 
I have the "how to keep your corvair alive" book, but the section on brakes 
only tells you what pads to get and how to grease your wheel bearings.

So my question is: what is the likely culprit? Master cylinder, wheel 
cylinder, lines?
Other info: P/O replaced 4-bolt master with two bolt late car master. Drivers 
side rear wheel is locked, even while going through gears on blocks 
(passenger side wheel is turning), these brakes worked previously, and there are no 
visible leaks.

I already know some of you are going to tell me to replace everything, 
including the lines and the drums. But I would rather root out the specific problem 
so I can learn something.

I'm a carpenter, not a mechanic, so forgive me if these are seemingly simple 
questions!