OK, posting this now to see if I am headed in the right direction, going to the city, will not be back until tonight, when I will have someone here to help me bleed the system.

Pulled my 300 out of where it has been sitting for about 5ish years. Front tires were flat, and when I swapped wheels, saw the calipers were rusty bad, and when I went to move, were seized on. As it moved, they broke free, but as I want to use it for the winter, and have the parts, decided to do the brakes complete anyway.

Pedal was SOLID no movement but if I pushed hard going real slow, the car did stop.

Got it in the garage, pulled the calipers and rotors. Did the bearings, seals and replaced the calipers and pads. Hoses looked ok, but I do have some, may just change them.

Got in the car to bleed, pedal was still solid...pushed real hard and "bang" it broke free and went to the floor, which is where it stayed, until I manually pulled it up. Went through the fsm troubleshooting and it led to booster failure. I have one in the same year Magnum, so I performed the swap, but it still goes to the floor and stays (brake light on).

I performed a few tests, there is plenty of vacuum from the engine, so I thought check valve. If I go to start to pull it off, it leaks air in out the side... so I assume it is ok. I pulled the one from the Magnum and it is OK for sure, could swap. Took off the master again, got the car running, and the pedal still goes to the floor and stays, I thought that with the master off, the booster itself would hold it up? Took a medium phillips as per fsm and pushed in the piston on the master, it goes in nice, and returns, so tried with the front cap off, and I see fluid erupting from below.

Now I know there will be air in the lines at this point from the caliper swap, could this be the only problem? When I get home tonight, I will bleed complete, starting at the right rear. In that, if my boy is pushing the pedal down (open bleeder) then I close and he manually pulls up the pedal...by rights it should start working on its own properly as air gets out of the system, yes? Pulling it up should not cause a problem in the bleeding process?

I will do the rear brakes, I do have all the hoses if need be and I do have a rebuilt m/c I can bench bleed and try.

Any and all suggestions appreciated, no matter how simple or stupid it may seem, getting frustrated at this point.

I would suggest looking for a blown brake line or wheel cylinder, caliper etc.What you describe is normally caused by something in the system being open. It could be from the lines being empty but it never hurts to do a close look over of the whole system.

Shall do, the front lines look ok with no seepage, and I did put on the new calipers. Will inspect the rears tonight...maybe I have some wheel cylinders here too.

No leaks in the system that I see. I swapped in a known good booster. Check valve is good. New calipers and pads, new shoes and wheel cylinders, bench bled new m/c, bled all brakes all seems good that way. Same problem. ugh.

Pretty sure it is in the pedal linkage, going to lubriplate it all and see what happens. After a drive and stabbing it hard a few times it comes back 95%, and the last bit is pedal play only.