I'm not familiar with the cap on your '59 master cylinder, the one on my '55 is metal and a 7/8? socket fits it. Some times you can use a punch and hammer to get the cap turning. If the brake fluid leaked out of the master cyl, then it is definatly bad and needs replaced/rebuilt. Check the fittings and lines for leakage also. And if the cylinder is empty it will need to be bled when you fill it up.
