if you had a "catastrophic" fluid loss it should be easily visible someplace, but it may also be that you didn't know the fluid was very low until it got to the point there wasn't enuff to keep the pedal up - check the master for level, refill and pump for awhile and see what happens
look all over underneath, check for a blown rubber hose .... check the fronts first, pull the adjuster plugs in back and shine a light in to see if it looks gummy .... if it lost a lot of fluid via a back cylinder there should be evidence of it around the drum edges at least
Bill