Nurse! We got a Bleeder...

lately I've noticed I'm having a bit of a brake pressure bleed off and I'm not really sure what to point my finger at and talk dirty too. I have a 73 D100 with manual brakes that I personally have never tampered with. Just recently "within the last 4-5months" noticed while sitting at a light with my foot on the brakes the peddle "slowly" begins to go to the floor. Honestly most of the time I didn't even notice, until lately my e-brake light started coming. lol had to google the issue turns out its also an "idiot" light for braking system. I'm the idiot. So like most people I started looking for a leaks and all four corners are dry and no wet spots or leaks in the line system and all pads for both front disc's and rear drums are new within the last 3yrs. Checked the master cylinder and its completely full, so now I'm questioning the Master cylinder which bring me here to ask the question. In all the cars I've owned over the years I've never had a master cylinder give us the ghost and I'm not sure what to look for. I'm guessing the plunger is worn or torn and its allowing the breaking pressure to bleed off back into the bowls which is why I'm having no leaks but loosing pressure.
Am I on the right track or am I missing something that might not be obvious to me at the moment?