I did the disc brake conversion on my 68 about 3 months ago. Everything worked great until about 2 weeks ago. I was driving along and went to brake, i noticed a lot of travel in the pedal. It got worse to the point where the fronts would not engage at all, only the rears would lock up with the pedal pressed all the way to the floor.
I asked what the problem could be and some said it was the MC. I figured i would bleed the fronts once before buying a new MC. THAT fixed my problem, i saw a lot of foamy fluid come out. They worked great up until today, started to feel the pedal getting muchy again.

What could this be?!

Get under the car and look for seepage at all connections, calipers, hoses, and steel lines. Whatever isnt new is suspect. Did you lose any fluid? If the fluid level is ok when you lost pedal pressure and there are no seepage spots I would go with the master. Also if the master is shot you may be able to pump the pedal a few times and feel pressure build back up.

Check for fluid at the master cyl to booster connection.Frequent area for fluid to leak and air to enter.