Typically power brakes are assisted by a vacuum booster.
When you press the brake pedal a small valve is opened which allows engine vacuum reduce pressure on a diaphram inside the brake booster assembly.
This diaphram assists you in pressing the master brake cylinder push-rod.
If this diaphram has leak, air will be drawn into engine. This then changes the fuel air ratio (lean) in the engine causing it to stall.
A way to test this is to disconnect the vacuum hose leading to the brake booster and plug the line. For safety reasons don't drive the with this hose disconnected.
Start the engine to see if the stalling problem is gone. If so replace the booster.