[Mgs] Brake failure - What caused it?

[Charley & Peggy Robinson]

  Here's a little trouble-shooting exercise for ya, gang.  I got my '69 
B out of the garage after it had been sitting undriven for several 
months.Backed her out, hit the brakes and the pedal went to the metal!  
Wound up on the grass next to my driveway, fortunately an uphill slope.  
So I look in the master cylinder and the front chamber is almost dry - 
my bad, should have kept it topped up.  So I fill it with Dot 4, do a 
little bleeding and get a nice firm pedal with no leaking down.  Drove 
the car a few times in the cool of the morning with no problem.

Couple days later I'm pulling slowly into my garage, got to stop and the 
brake pedal goes to the metal!  I almost yanked the handbrake lever off 
the floor!  So after I changed shorts, I looked in the master and it was 
still full.  Hopped in the car and tried the brakes again and they 
worked fine with a firm pedal.  Brake lights work too and there are no 
discernible leaks.

I think I know what's wrong.  What's your take?

CR