I am having problems with the master cylinder on a 1973 vette. When bench bleeding the master cylinder the fluid for the front brakes flow from the point that I start pushing the cylinder to the end. The problem is with the back brakes. The fluid stops flowing about 1/4" from the end of the cylinder push. This is not allowing pressure to apply the back brakes. The master cylinder is in good condition with no pitting and a new rebuild kit has been installed. I checked a factory-rebuilt master cylinder and it had fluid flowing for the complete push of the master cylinder. Any help would be appreciated.