master should shut off all electrics
I'm a pilot, not a mechanic, but I'm certain the master switch in a C150 should shut off the alternator and remove power from all electrics except the clock and the hobbs meter. I have 500 to 1000 hours in the C150.
It is common to see the red voltage light illuminate immediately after start. In later models, which have a separate switch for the alternator, this problem is eliminated by not selecting the alternator on until after start. In some airplanes it's labelled the "overvoltage" light, because the alternator shuts down automatically in the case of too much charge, and then the light illuminates. In either case, it tells you the alternator is offline.
I have a pilot's manual for the version with the split alt/bat master, but not for the one with the single switch. It has only a very simple circuit diagram. I'll bet if you first remove the fuse for the alternator, the electrics don't keep running when you switch the master off. (I'm assuming you have the old kind with fuses and the single master, and not the split master plus CBs, because you said "master" not "battery side of the master.")
I would not consider it safe to fly a C150 with the master switch malfunctioning as you describe. In-flight electrical emergencies require that the pilot be able to select electrics off with the master switch.
I can't imagine that's any help, because the big book from Cessna must give you more info than that, but whatever it takes to keep the magic smoke in the radios and out of the cockpit.