My brakes are locking up after i drive my car around 20 min. I have replaced everything from the master cylinder to the calipers including hard and soft lines. New fluid and bled them thoroughly. When it cools off the calipers start releasing. It is a 2002 hyundai accent. The only thing i havent replaced is the booster and proportioning valves. I need help!

If youve replaced all those parts start by checking everything is mounted correctly. If it is even a tiny amount out it will cause issues.
Next Re-Bleed the brake system starting with the caliper furthest away from the master cylinder and do the nearest one Last.

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

Does it pull to one side or the other when rolling? If so, then you may have a stuck caliper. If not, then you likely have a bad master cylinder and need to replace. If it is a stuck caliper, it would be wisest to replace BOTH calipers. If the brake pedal feels hard when you push, that would be another indicator the master cylinder is bad - but you may not have the hardness in the pedal. If pulling up on the pedal relieves the problem, I would suspect the master cylinder first. Driving on it could damage many components, and replacing them all may be expensive - I recommend verifying the master cylinder is bad and replacement before driving!!

Does your car have disc brakes around? rear caliper pins are common to seize, when rear brakes are replace is a good practice to have your shop replace rubber boots and clean pins on a wire wheel to remove all rust, also use a honning tool to clean caliper slides holes to bare metal assemble with a good quality brake lube. Caliper brackets(holders) also have to be clean by removing brake pads clips, clean back and front and grind off all rust from braket. this is one part may help, but have some concerns with your brakes lacking up after driving for a while, this is a symptom cause by a miss adjusted brake booster rod, when ever you replace a master cylinder you have to check the leng of it or your brakes will lockup somethimes so bad it won even move, have your shop set this to specs. good luck

Good question, on the antilock system is a bleader too and bleade it the same way you bleade the caliper. After all is done adjust the rear brakes, they should have a drag to them.

I had this same problem on my 90 Jetta but it was the master cylinder that was the problem, I first replaced the master cylinder because it was leaking badly. The new master cylinder I put on was a manufactured defect, the internal piston inside the master cylinder was not machined enough (10mills too long) so when you applied the brakes the piston would not move far enough to release the brakes but after 20 mins they would slowly release themselfs just as you said your brakes do. The only thing that can cause this problem (if you have changed everything) is the master cylinder or the brake pedal itself, did you do anything to the brake pedal or near it to change its movemment ? Make sure the brake pedal once depressed returns fully to the float position where it should be. I would try another master cylinder preferrable a used one from another vdub the same as yours, eg year,model and country of origin some parts are different if it was made in Mexico or Germany. Its going to be a pain in the *** to have to replace the master again but I see no alternative. There is one more thing you can check I just thought of....check all your brake lines from the master to the calipers for any signs of dents or sharpe curves if the metal brake line is bent too much it will restrict the flow of the fluid returning to the master cylinder after brake pedal release but not during braking. Good luck

Check the left rear brake for sticking on also, if it is then you have a master cylinder problem. replace brake master cylinder but check the adjustment of the brake light switch first. the switch is above the brake pedal, make sure it is not holding the pedal partially on.

sounds to me if you replaced all of that than the power booster is sticking

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

u need to bleed ur brakes some more Helping one person at a time Shaun