Brake issue on the 69

[Kw69]

Hey guys I had all the brakes done about a year ago and replaced the rubber lines. The metal ones are still original. Truck has non power assisted drum brakes. Anyway the last few weeks I have noticed that the pedal would lose pressure, like I had air in the system. But I should not have any air cause I can't find a leak and they were bleed a year ago. Funny thing is you can pump the pedal after you lose pressure one time and you have pressure again. This symptom comes and goes a few times if you drive it. So I checked the fluid and its full! Anyway being the original brake master cylinder, I'm thinking it's going bad. After all it's from 1969 so I figured it's time. Does my thought sound right? Or am I missing something ? If I replace it should I also just go for the power assisted as well? And if so where is a good power unit available?mauto zone, advanced auto, or ? Or would it be worth to convert the whole truck to five lug and disc? eBay about 1100.00 ...... Seeing what you guys think. Thanks in advance