Brakes wont get hard after several bleeds

Assuming you are bleeding in the correct order, usually furthest first, closest last, then perhaps you need to go to a shop that uses a power bleeding machine - some vehicles are very challenging to bleed.

As soon as I replaced the rear, the pedal would get hard during bleeding, but as soon as I started the truck the pedal falls to the floor no matter how many bleeds I perform.

This happens because of power brakes. The pedal is hard when there is no power assist. When you start the engine, the power assist starts. Then the pedal will become more "flexible" due to the power assist doing most of the braking. So if you stand with your whole weight on the pedal with the power assist on, it could go to the floor, especially if you are heavy like me and if there's still some air in the system.

I suppose your bleeding still leaves some air in the system. Either that or then you are pressing the brake pedal too hard with the power brakes on.

You could of course do a very careful drive in the car. Are the brakes powerful enough to brake in an emergency braking situation? Do this in a road that has no traffic, as you cannot be certain about the quality of your brakes. It is preferable to test the brakes first at a very low speed, 5 km/h or so, and then gradually increase your speed, but not too much or else you can cause an accident due to poor brakes. You should be able to lock the wheels. If not, there's something wrong with your braking system or the bleeding procedure.

Nevertheless, even with properly bled brakes, you will note the pedal deflects far more with the power assist on than it does with the power assist off. I have seen this effect on all of my cars except the last hybrid car, where opening the driver's door causes the brake vacuum pump to operate so there is no conceivable situation with me in the driver's seat and power brakes off.