Brake on front passenger side continuously locks up (seizes). Have replaced the master cylinder, rotor, pads and caliper but problem persists. System was bled entirely and new brake fluid flushed througout the system and brakes seized again, only on right side. What could it be?

Air in the system is what usually causes a caliper to lock up when everything else is functioning properly. If you can't get the air out of the system you will continue to have issues because when the brake system starts getting hot, the air expands inside the brake system causing the weakest brake to be applied. You will need to bleed the brakes once you fix the caliper that is not able to be bled. Remember, bleed the brakes from the farthest wheel from the master cylinder first and then continue to the nearest wheel last.

probably none of the above
have the brake master cylinder replaced first
then work on the pins that allow the calliper body to self center
probably seized on the pins holding one pad on the rotor

Sounds like your master cylinder is not working properly, so you may have to replace it as there does not seem to be the correct amount of brake pressure that gets to the rear brakes. Are your rear calipers or brakes seized, to they move freely. Is the parking brake cable seized and causing the rear brakes to not function properly. Lots of thing it could be.

I think the problem is the brake pad or caliper, brake pad are too thick or the caliper are not good casted.
First try to confirm if you get a right brake caliper ,then confirm the brake disc ,brake pad, if they all ok, tear down the bracket of caliper, put into brake pad, then put into brake rotor.
http://www.fiverhope.com/category-4-b0-Brake-caliper.html

Where to start. . . .
A defective hose would only affect that wheel.
A faulty booster or master cylinder could cause the front wheels to lock and at varied amounts. You may have a metering block or just a junction block for the front lines. It would be mounted near the frame if you have one. Some Ford models from 88 - 92 used a master cylinder with the metering valve built in.
Its possible the front calipers got hot enough with the old pads to damage both front hoses.
I would want to know what it takes to release the pressure on the pads. Do you have to take the hoses loose, or just the bleeder screws on the calipers ? Or are the calipers not sliding back and forth ?

Problem is in your master cylinder assembly. It may be an obstructed compensator, piston seized in bore, or deformed piston cups. Have also your brake lines checked for obstruction.

The hose could be collapsed internally holding residual pressure and keping the pads applied. Also make sure that the caliper is sliding free side to side.

Why not buy used part?

YOU NEED REMOVE A LITTLE BRAKE FLUID FROM MASTER CYLINDER.THEN YOU RAISE VECHICLE SAFELY.SUPPORT IT WITH JACK STAND.MAKE SURE VECHICLE IS ON LEVEL SOLID GROUND.REMOVE TIRE OR WHEEL.THEN YOU NEED A LARGE C - CLAMP.PLACE C - CLAMP OVER BRAKE CALIPER AND BRAKE SHOE BACKING.TIGHTEN C - CLAMP JUST ENOUGH PUSH CALIPER PISTON BACK IN CYLINDER BORE TO REMOVE CALIPERS FROM ROTORS.TO REMOVE BRAKE CALIPER YOU NEED 2 OPEN AND BOX END WRENCHES ONE TO HOLD CALIPER SLIDE PINS AND THE OTHER WRENCH FOR LOOSEN CALIPER BOLTS.THEN YOU REMOVE CALIPER AND REPLACE FRONT BRAKE SHOES.IF YOU NEED TO REMOVE BRAKE ROTORS JUST REMOVE THE BRAKE CALIPER CARRIER SUPPORT BOLTS THE REMOVE THE BRAKE CALIPER CARRIER SUPPORT THEN REMOVE ROTOR.YOU NEED TO REPLACE BOLTS WITH NEWS AND USE LOCK TIGHT.THEN YOU PUT IN NEW BRAKE PADS IN CALIPER PUT CALIPER BACK ON SUPPORT CARRIER.REPLACE WITH NEW BOLTS USE LOCK TIGHT THEN TORQUE BOLTS TO 26 FT LBS.MAKE SURE MASTER CYLINDER COVER IN PLACE.WHEN YOU FINISH REPLACING BOTH FRONT BRAKE PADS PUMP BRAKE PEDAL TO SEAT BRAKE SHOES AGAINST BRAKE ROTORS SO YOU WILL HAVE FIRM BRAKES WHEN YOU APPLY THEM. THEN CHECK YOUR BRAKE FLUID LEVEL IN MASTER CYLINDER MAKE SURE ITS NOT LOW. MAKE SURE YOU HAVE FIRM BRAKES BEFORE DRIVING.IF YOU HAVE ANY PROBLEMS CHANGING FRONT BRAKE PADS YOU CAN GO TO ANY AUTO PARTS STORE AND BUY A HAYNES REPAIR MANUAL.

Ok Have you recently replaced the pads? Do they act normal after they cool down? Are the pads in good condition, pads that are so worn that the caliper pistons come out beond their normal limit may stick there due to excessively worn pads. If so make sure they are at least factory grade quality Good are bendix, the only thing that can cause your symptoms is a bad master cylinder because it affects BOTH front calipers equally if it was a caliper it is likely to affect only ONE. Let me ask you this happens after driving in heavy traffic or after along drive?
If So there have been countless cases of boiling brake fluid, Old Brake Fluid gathers moisture and its boiling point is lowered, Fluch out ALL the old brake fluid and have new put it.

also is there any chance even the slightest chance that something other than brake fluid even a drop or two is enough to do damage. has been added to the master cylinder at any time? If so you need to replace EVERY PART IN THE BRAKE SYSTEM INCLUDING THE ABS CONTROLLER THAT HAS ANY RUBBER PIECES IN IT CALIPERS HOSES WHEEL CYLINDERS,MASTER CYLINDER just for your information.
I believe you have a bad master cylinder but do not buy a rebuilt one only bran new and preferably from the dealer.
Please contact me if you need more help and would you give me a good rating if you feel my information is helpful.
Thank you ,Randy