1996 Ford Transit sudden brake loss?

Here is the scene EXACTLY :

I start up my diesel Transit and drive down the steep(ish) 30m drive. Brake at the bottom (no prob). Along the flat to next junction 200m, brake (no prob). Pull out and have to make a turn almost immediately needing brakes again.... None there !!

When I say 'none there' specifically i mean, the pedal travel is unrestricted more or less all the way to the floor. There is no resistance on the pedal, and only the very last inch has any braking effect. I come to a halt and check under bonnet for loose servo pipe, lack of brake fluid etc... But everything OK.

I turn around and retrace my journey, testing he brakes with my available left foot as and when... still nothing....
I turn up my steep(ish) drive requiring first gear, and swing the corner at the top doing about 5 mph. Push the brake pedal instinctively and..... IT'S ALL THERE AGAIN !!!

What can have happened - can it happen again ???

Many thanks