Intermittent brake fault

[magicdeo]

Gday. Ive got a 97 2.8 rodeo dual cab. The brake pedal has gone almost to the floor 3 times in the last 4 days. Apart from the 3 times, the brakes have been working as per usual. Im thinking master cylinder? I assume a fault with the booster or vacuum pump would always be preset, and any issues with seals on calipers or wheel cylinders would be an external leak? Any ideas would be good thanks