brakes question

I'm hoping someone can give me some advice on how to handle a problem with my car.

96 Camry.
Scenario is sitting in standard, stop and go, traffic. I apply some pressure on the brakes. Enough to stop the car. About 10-20 seconds later the brake pedal feels like it is getting pulled out from under my foot. I keep the pressure on the break and follow the pedal down. After a few seconds of this the car starts to move forward. I can then increase pressure on the break pedal and stop the car.

I took the car to the dealership and they said everything looks fine. They flushed the break fluid and that seemed to help for a short while (few weeks maybe) but now it is happening again.

Thoughts?

Thank you

sounds like  master clinder or brake booster could be faulty. perhaps a leak in a vacuum line somewhere too

Thanks. It was the master cylinder. All good now!