1985 Dodge D150 Brake Problem

I have an 85 3/4 ton 4wd that occasionally the brake pedal goes all the way to the floor. There is always a tiny bit of rear brake at the very bottom. Pump it a few times and it goes back to normal and has excellent brakes until the next time it does it.

I have tried everything i can think of and it still does it. All new wheel cylinders on the rear, brake shoes are adjusted correctly, new calipers, all 4 rubber hoses and all suspect hard lines changed, several master cylinders tried, different booster. I have bled a couple gallons of fluid through the damn thing trying to figure it out. No leaks anywhere, I get all the air out and i know how to bleed brakes... I have successfully bled brake systems for decades and know how to do it.

any ideas? im at a loss