Sudden brake loss in a 76 300D

[Sterling30]

The brakes have always been fine on this car till last night. The car has been parked the last 3 weeks ans when I started it the brake light was on and the pedal went all the way to the floor. I was able to get it into the garage where I topped off the brake fluid, (it did take most of the contents of a small can) that fixed the light indicator but I still had no brakes.

I saw no leaks from the master cylinder or anywhere else but the pedal would not pump up properly and hold so the brakes seem completely gone for the moment.

Anyone with any ideas what could be wrong?