I replaced the front brake pads and the two front calipers on my '99 Nissan Altima (manual transmission). After bleeding the front lines, and everything seeming to go like it should, I turned the car on and the brake pedal went all the way to the floor. After pumping several times, I was able to get some resistance, but nothing that felt the way it should. We then bled the lines again, this time on all four, and nothing changed. Bled the front again, nothing happened, and I had a mechanic look at it and bleed all four lines again, still with no result. I was able to drive it around, and I can get it to brake and stop, but it certainly doesn't brake as well as it should. I have heard that it could be the master cylindar, or it could be the brake assist assembly behind the cylindar. I want to save money on this as much as I can, so I'm wondering what the best way to get this diagnosed and fixed would be. Can the electronic tests that the guys at Auto Zone detect this? Is there any visible symptom I should see when I check under the hood? And most importantly, is my car safe to drive?