Just remembered running across this yesterday:
"Does your 4Runner have rear drum brakes instead of discs? If so, worn or out of adjustment rear brake shoes can cause a low pedal and even mimic a failing master cylinder."
Low brake pedal after new pads and bleeding?
I just installed the brake shoes yesterday. Cleaned and lubed the threaded self-adjusting mechanisms, and adjusted to just allow the drums to be installed. I figured that the self-adjust mechanism would fine tune it. I didn’t turn the drums and the shoes need to wear in.
After pedal sinks, one or two pumps brings it right back. And as I mentioned in another reply, the pedal never loses braking power. It sinks just so low and no lower.