85 C3500 brake issues

[brothers69c-10]

So heres the scoop. I was drivingmy 85 crew cab last week and noticed the brake pedal fading more and more. I checked the reservoir and the front compartment was empty and the rear was full. There has to be a leak somewhere so I checked all the lines from front to back and they were all dry. I replaced the right side line that runs along the rear axle anyways as I did the left side a couple months ago. Filled the reservoir and bled the brakes. The first pumpgoes clear to the floor, bit will pump up on the 2nd and 3rd pump. We bled the brakes for 30 minutes and nothing changed. We were bleeding the brakes at the lines because the bleeders were rusted stuck. I decided to change out the wheel cylinders so I could bleed the brakes appropriately. Did that and after bleeding for quite awhile still the same results. Next decided to change the master cylinder. Bench bled that and bled the brakes again. So basically everything has been replaced and I cannot find any leaks anywhere. So when you go to stop the pedal at first has a little bit of pressure and you feel the brakes grab right away and then let go and you can continue topush the pedal down clear to the floor before they grab again. Yet you can pump them a couple times and then they're like normal. I should add that this truck has hydraboost. Anybody have any ideas?