Hi John, for what it's worth......... Although a SEAT my Terra van has always had appalling brakes. Loads of travel and braking only just off the floor. I've tried vacuum bleeding, pressure bleeding and good old get the other half to pump the pedal bleeding.
I've fitted a new servo, master cylinder, discs, pads and removed the rear brake balance valve.
On the back I fitted 'injection' self adjusting rear shoes and....... The brakes are crap.
My next idea is to fit engine, axles and running gear from a Ford Focus. That should do the trick.
I have a Sisley with exactly the same problem, scared me to death when I first bought it, went to pull up at the first 'T' junction having left the previous owners house and nearly sailed across it. The pedal went almost to the floor before any apparent braking so need to sort it before the MOT.
I plan on changing pads, discs, rear brake shoes and all flexible hoses and hope this resolves the problem.