1988 Tercel Brake Issues

i BOUGHT A 1988 TERCEL FOR 250. WHEN I BOUGHT IT I REBUILT THE BRAKE CALIPERS . THEY HAVE BEEN WORKING FINE EVER SINCE BUT KNOW ALL THE SUDDEN. O RUN THE CAR TO ANYWHERES AND AFTER ABOUT 5 TO 10 MINUTES OF USE . THE BRAKE PEDAL GETS ROCK HARD AND WONT LOOSEN UP TO THE POINT WHERE THE CAR WONT HARDLY MOVE CAUSE THE CALIPERS ARE BRAKING BUT WONT RELEASE. I MEAN ALL I KNOW IS THAT WHEN THE MASTER CYLINDER GOES BAD THERE IS NO PRESSURE AT ALL. BUT IF ANYBODY CAN GIVE ME SOME ADVICE PLEASE FEEL FREE TO DO SO . THANKS.......LA SOMBRA:nono:

Bad flex lines can cause blockages that won't allow the calipers to release.