Brake pedal sometimes goes to the floor

Sometimes when I need to stop quickly, the brake pedal goes to the floor and I have to pump it which is dangerous. It does not always happen. I have taken it to several mechanics for this problem and we have changed the master cylinder, rebuilt the booster and machined the discs and replaced the pads. Nothing has helped. Even at the best of times, the brakes are not nearly as good as they used to be. It would be impossible to lock the brakes and it does not have ABS. I would like to sell this car but must fix this problem first.

Where is the proportioning valve? My biggest problem is that I am a retired American living in Mexico and the mechanics here are always trying to do things the cheapest way possible with little knowledge and/or equipment. If I can find the proportioning valve, I can show it to them and ask them to bleed it first.

The Stratus does have four wheel disc brakes and for that reason, I did not think that it had a proportioning valve. I thought perhaps that the discs were too worn but do not have a micrometer.