Very occasionally (in other words, it'll never happen while an Alfa mechanic is sitting in the car ), the brake pedal 'sinks' about 1cm during braking. This doesn't have to be hard braking, just steady braking. When the pedal has reached its new level, braking continues. What on earth is that likely to be? Master cylinder, distribution valve? It feels like something is giving way somewhere in the hydraulics. I've just had new pads and discs fitted, it did it before and after this, so nothing to do with them, methinks. Ho hum
Unless there's some other factor (ABS bypass valve or something...?) the symptoms sound like weedy master cylinder o-ring seals. If you lift your foot off the brake and re-apply them those symptoms will repeat themselves.