I have an 88 4Runner and out of the blue yesterday the brake pedal starts to drop if I maintain pressure on it. It stops great initially but if I'm on a hill or just hold the brake it will drop all the way to the floor within about 30 seconds if I don't pump it. There is no leak anywhere.........!?!?!? Any ideas?? Thanks for your time.....................TIM

 

The seals are going, probably in the master cylinder. It doesn't leak, just the fluid gets past the seals and you loose pressure. Replace or rebuild the master.
On an 88 if the slave is original you might want to replace that too while you're there.