Brakes started at floor first time you applied them

The guide help me fix it thank you.

My brakes are going to the floor and truck is barely stopping. I replaced the master cylinder, bench bled it first then installed it. Bled all the lines working from the furthest ( rear passenger) to the front. Made sure clear fluid came out with no air, pedal was firm until I started it. Then it went to the floor again. I cracked open the ABS actuator lines and bled them ( in lines first the exit lines ). What AM I DOING WRONG? How do I bleed the ABS? Please help.

I did all you suggested, once engine is started it goes to the floor, pumping brakes has little or no difference, I think the ABS actuator has air in it. I need a way to open the valves to properly bleed them. Any Ideas?

I had changed my brake pads about four or five months ago and when I did my piston literally fell out and I believe it was because the c-clamp I used got stuck in the hole of the piston. I then put it back together but noticed a small leak. Well I didn't repair it at the time but was never a big issue. About every other week I had to refill brake fluid. As time went on I had to refill more frequently. So about a month ago my brakes started getting worse until they finally just quit. I decided to bleed them. That back brakes were fine but not the front, pedal goes to floor and won't pump up. Traced it all the way to port on the master, not getting any brake fluid out of the one port for my front brakes. Did a bench bleed and both ports worked fine, put back on my truck and had same issue. I then called everyone and nobody knew so I went and got a used one, put it on and got great brakes. Well, here it is about a month later and now I'm having the same issue again, pedal to the floor. Yesterday I bought a rebuild kit with seals only but still the same problem. Why?

Well it's just strange how my original master cylinder and the used one I purchased done the exact same thing. Speaking of the caliper, I replaced the seals.