Out today for a routine pootle around my local and very muddy woods and I was literally just riding along when my front brake began to feel very spongy. I stopped and tried the lever a few times and it became progressively softer until it came back to the bar. Some fluid appeared to leak from the lower reservoir and there was no pressure on the lever. The lever piston moved but there was no resistance and the calipers did not respond. When I got home I checked the reservoir and there was still quite a lot of fluid there. I attempted to bleed but could not get the fluid to pump - the lever had no effect when depressed.
Any ideas what could be the cause?