Dash-8 Captain reported after parking; the parking brake released and aircraft rolled several inches before stopping. First Officer discovered their brakes were working properly.

Narrative

After parking on spot 1 and when we were in the midst of our shut down flows; the parking brake released. I had my feet on the brakes on the rudder pedals; but apparently not enough as the aircraft rolled a couple of inches. In fact; when it happened; I thought to myself; 'Good thing I have my feet on the brakes.'; and wasn't aware we had rolled at all until my First Officer asked; 'Are we rolling?' By the time she asked; the aircraft had stopped and she noticed her brakes were ineffective as my feet were still holding pressure on the brakes.