Very similar thng happend on my old mondeo, dropped some freinds off at the pub, then as I were about to go home, and the brakes didnt feel "right", and the warnig light came on the dash. So instead of going down a 1 in 4 gradient 'S' bank on the side of a cliff (seriously) I just went to the supermarket carpark and got my mum to pick me up.
Next day, put a full bottle of brake fluid in everything was ok, and went the long way round to get home (5 miles instead of 2) and the resovier was empty
. My neibour came over to have a look at the brakes with my dad, they couldnt tell what it was, and I noticed that the pipe had a very small gash in it.
It was probably caused earlier by the imbusils at "bang it and bodge it motors up the road when the did the handrake cable (apparently, it failed the MOT a week later on the handbrake) amd the rubber pipe was rubbing on the suspension spring.
£37 to fix, and a top up of brake fluid.