brake booster and m/c, or just m/c?

[doofus]

I’ve read some of the threads about brake fluid getting sucked into the master cylinder, and I suspect this is happening with my recently-acquired 300TD. I filled the reservoir several weeks ago when the red warning light began to flicker-the level was just low enough to make it come on I guess-and all was ok until today, when it came on again a few blocks from work. Then at the same time the brake pedal got very hard and I had no power assist so I had to really stand on the brakes to stop the car (this was two corners from the parking lot, at low speed). I pulled into the lot and the car wouldn’t shut off, so I used the lever under the hood (which worked instantly).

My questions are (because I’m confused by the various threads I’ve read): am I correct in assuming that there’s a connection between the sudden loss of power-brake assist and the inability to turn the engine off with the key? and do I need to replace only the master cylinder, or the master cylinder AND the brake booster? thanks for any advice!