If the pedal dropped very low , this is a sign of a bad master cylinder.

Look for a cut away pic of one on the net. The rubber seals have a lip on them that faces the pressure side. As the seals wear, the lips are not under as much compression and will leak ( internal to the MC ) . This causes loss of braking pressure.

Why are the brakes seemingly OK now? If the pedal is pressed rapidly the lips fan out and are pushed into the cylinder walls making for a good seal.

The easy way to test a MC is to place your foot lightly on the brake pedal and as it falls keep following. If the MC is leaking internally the pedal will slowly fall to the floor. Sometimes in the early stages of failure you need to drive on a slightly bumpy road to generate some vibration,

How many lines do you have coming out of the MC? If only one it is a really good idea to convert to a dual circuit master cylinder. This will at least give you 1/2 braking if something fails.

If you do have a dual circuit MC , you still had 1/2 braking ( front or rear ) and a very low hard pedal.

If the pedal remained high and was hard to press, you have a brake booster / vacuum supply problem. In that case pressing harder would have stopped the car.