Why does master cylinder keep going bad?

First look for signs of leakage. Is the master cylinder mounted properly and the nuts fastened tight? Is the master cylinder properly bled. If not, put the master cylinder in a secure device like a vice. Fill the master cylinder full of brake fluid. Use a flat blade screwdriver and push the piston in to let air out. Have an assistant hold the two openings tight with the thumbs/fingers. Release the tension on the piston with the screwdriver, then remove thumbs/fingers. Repeat procedure until no air is seen coming out. Install master cylinder and bleed both lines. Make sure all the air is squirted out.