I just went through a similar situation. I lost the rear brakes on my 88 runner. Master cylinder was bypassing on the seal for the rear circuit. I never lost a drop of fluid out of the master cylinder. One day fine, the next day no rear brakes, and very spongy pedal. Rebuild/replace the master cylinder.

1988 4Runner V-6 Auto, Locked out back, LSD up front, some lift, some tire, and a casual attitude towards body damage.