It sounds like your brake is going into hydrostatic lock due thermal expansion because the feed port from the reservoir to the master cylinder is possibly blocked not allowing fluid to go back into reservoir when brake lever is released . Get a syringe with a clear plastic/rubber pipe fill it with DOT 3 brake fluid ,connect it to bleed nipple on caliper ,crack nipple open and pressurize using syringe. If the reservoir does not fill up, the master cylinder is at fault (piston is blocking supply/fluid return passage to reservoir or foreign matter is blocking orifice. Pull master cylinder apart and investigate. Start dis- assembly by removing reservoir and you will see the port in question. If it is covered up when brake lever is in relaxed position then there is your problem.You will have to investigate further by complete dis-assembly.