The brake booster is the big round thing between the MC & the firewall. It uses intake-manifold-vacuum as it's 'boost'. If it's leaking or not adjusted correctly, it lets too much air into the intake manifold.

Stopped, engine idling, let everything settle down for 30 seconds. Without touching anything else, pump the brake pedal several times. Engine RPM should increase somewhat, since the brake booster is letting extra air into the engine. But it shouldn't be so severe that it stalls the engine.