Master cylinder seems to work good
My brake calipers won't pump equally or won't work at all replaced one on 2000 Nissan Altima
Has all the air been bled from the system, are the brake lines bad, maybe the master cylinder is a problem if they sometimes don't work at all.
Caliper not sliding/frozen on the mounting hardware, common on this vehicle! They have to move as pads wear. This will result in low pedal. Can be tested by clamping flex brake hose, then trying brake pedal.