My 1994 Accord recently began demonstrating a spongy or mushy brake
pedal when I'm stopped. When I apply the brakes it works OK and
momentarily holds firm as I push on the pedal to hold the car, but
them it becomes mushy and I continue to push down until I almost bury
the pedal in the firewall. The fluid level is up and there are no
leaks visible at the master cylinder, in the lines or at the wheels.

What am I missing?