Holden Commodore 2001: Brake master cylinder problem

On two occasions recently, while driving our 2001 VX Commodore Series III along the freeway the brake pedal travel has suddenly increased to about half pedal. On both occasions the "Park Brake/Brake Fail" warning light illuminated followed by a warning chime. The brakes felt normal except for the increase in pedal travel. After pulling over and pumping the brake pedal, the warning light went off and the brake pedal travel returned to normal. Brake fluid levels are normal and hand brake travel and function also are normal. Have you any suggestions as to what may be causing this problem?