Hi David
I'd have thought that an ABS problem would trigger a warning light or fault code.
Did this happen overnight or have you not tried it for a few weeks/months? If removing the caliper and doing a thorough bleeding doesn't fix it
http://www.ukmonster.co.uk/monster/s...ad.php?t=58090 then the only thing I can think of is a failed seal in the master cylinder - it might be tight enough to pump fluid but not a good enough seal to sustain any pressure.
Edit PS thanks, Rollo!