04 free star soft pedal has new master cylinder pads and rotors

disc brakes can only lock up because the fluid is not allowed to return to the master cylinder reservoir
That is normally because the compensating port in the master cylinder is blocked
You say that you have fitted 2 new master cylinders but did you check the brake pedal free play because if there is not enough free play then the piston cup covers the compensating hole and the problem that you have is the result

Have you driven it since the brakes where replaced?

Did you bleed all four wheels?

have you "broken in" the new pads?

Are the rears drum or disk? If drum are they adjusted correctly?

are you using remanufactured master cylinders? If so keep replacing them. eventually you will get one that works. New is pricey but a better way to go.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

If you replace the wheel cylinders or brake calipers and did not bleed them properly, you most likely have air in the lines, If the rear brakes are drum brakes they need to be adjusted (if taken apart or new parts put on) or the pedal may sink when pressed.
1. If the brake pedal is soft when pressed down, the brake lines have air in them and need bleeding to remove the air.
2. a soft brake pedal can also be a bad master cylinder. Replace the master cylinder if the lines have been bleed correctly but the pedal is still soft or sinking when the pedal is pressed.

Bleed with the car running and make sure all the air is out. Now make sure there is no leak and the pad contact are adjusted without much gap so that the calipers hold on. make sure the servo assist is correct, if so and the master cylinder was set right , it must work. Please recheck all your work once again to trim up.

no worn pads do not cause a sinking pedal, you have a defective brake master cylinder, the seals inside the master cylinder are leaking

Are the rotors worn? Worn rotors and new pads will sometimes cause this. The pad sits across the ridges and no matter how much you bleed the system the pedal just wont stiffen up. (See diagram. A - New pads, B - Rotor, C - Ridge)
If you did not install new pads you may also want to check the master cylinder seals. First bleed the system at the pipes that connect to the master. If the rotors are ok, you've bled the master cylinder and the problem persists then replace the seals or the complete master cylinder.

SOUNDS LIKE MASTER CYLINDER SEALS ARE GONE. REPLACE MASTER CYLINDER WITH NEW OR REBUILT UNIT.

Because of the ABS system if you press the brake pedal with too much pressure then it will give you the illusion of a soft pedal. Does it feel like a soft pedal when the vehical is in motion and you then press on the brake pedal (normal)? The other thing is if your pads and/or caliper were frozen, when you changed them out. The brakes are not going to feel or brake exactly the same, after the repair. But you are there, if the pedal is real low or goes to the floor then there is still air in the system or for some reason, your master cylinder is defective.

If they rebled the brakes and the pedal goes to the floor, chances are that the master cylinder is bad. Probably cost about $80-100 for the part if you could do it yourself

Your brake light switch is adjusted in all the way and is pressing on the brake pedal. This causes the brake to be slightly applied. As you drive the vehicle the brake fluid heats up and expands, the brakes will apply more and more until you see the smoke. Bottom line: your brakes are always applied at the master cylinder because something(usually the brake light switch) is out of adjustment.