Master cylinders, part deux

I seem to have a firm pedal on my front brakes, but a soft pedal on my rears [pushing the rods / watching which moves more on when pedal pressed] I don't see any leaks. I added an in-line T to put a brake light on my dash, and plan to re-bleed [again] with someone holding the T lower than the line, but I've bled this thing endlessly, without changing the result. Any ideas? I thought about changing the lockheed master cylinder [Swift DB-1], but I am not sure that is the problem, as there is no leak.

If I need to change it, how would I identify and get one? Moss list MC by type of car and year. No specs given.

Thanks,