What brand are you using? I've only had to fix my master once in 120,000 kilometers. If you keep buying a super cheap replacement part and have to replace it every few months, then it's obvious that either the part is not durable or something else is wrong.

Check the rest of the system, also... you might have a crimp in the hose leading to the clutch causing extra pressure. The reservoir being shared between the brakes and the clutch shouldn't be a problem, but if your fluid is degrading, then you ought to flush the entire system. Yes, it's okay to use DOT4.