honda 200x rear brakes

[atc90]

hi, can anyone tell me why my rear brakes pump up like a hydraulic jack?they lock after a few uses ,the only way to free the rear wheels up is to open the caliper bleeder valve for a second, then everything works freely again for a few times, I rebuilt the caliper and the piston moves in the bore smoothly I also installed new seals the rear calipers piston and bore look like new, the caliper moves freely side to side as I cleaned and lubricated the pins and put a new boot on the big pin. the rear brakes have no leaks, when the brakes lockup the caliper is tight on the disc and it is nearly impossible to pull the caliper from the disc after it has locked up. the rear master cyl looked ok when I dissasembled it for cleaning its piston moves freely in its bore, after the brakes lock up they dont seem to release if the atc sits for a while, the rear brakes had evidence of overheating in the past as the rear disc was warped and cup shaped, I replaced the disc with a flat one, the rear master cylinder piston looks like it is at the bottom of its stroke when the brakes arent being used where it should be ,however oddly the rear brakes feel like there is almost no cylinder travel before the brakes apply,the caliper is centered perfectly over the disc I have dis abled the parking brake until the problem is fixed,any ideas? thanks

[89TRX350RX]

I would look at changing the flexible brake line. I have seen it on trucks/cars where the front flex-line gets some type of deformity inside the line and allows the fluid through to the caliper under the pressure of squeezing the brakes/pedal, but will not allow the pressure to bleed back into the reservoir. The way you described cracking the bleeder screw, is the easiest way to diagnose it. You can also try cracking the line at the brake pedal on the pressure side of the piston. If the brakes don't release, then it is definitely the line that is damaged inside.

Let me know... Jamie