'88 240DL PB, no ABS

About a year ago, the brake pedal started to slowly depress until it hit the floor. I diagnosed it as a bad master cylinder (no fluid loss) and replaced it with a new unit. Now it is happening again, same symptoms (my daughter says it has been happening on and off for a few months now but she "forgot to tell me"). I suppose it could be that the new master cylinder has gone south but after less than a year?

So I am looking for other possibilities and thought I'd ask...could the brake junction block/failure switch produce these symptoms? Pedal slowly depressing to the floor (takes about 1 minute) with NO fluid loss anywhere and the brake failure light coming on once the pedal hits the floor? I'm thinking no, but...

Or am I just looking at another master cylinder?

I don't want to keep replacing master cylinders if the cause is actually something else.

Thanks...
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Bob K