Service brake efficiency, still can't fully explain the circumstances.
The problem appeared to be the rear brakes only.
All I did to pass MOT was to check pistons weren't sticking, new pads all round and a quick bleed the improvement was noticeable in the readings
Before 91,92 and after 116,117 (this was done on the roll road).
I'm going to speak to a very knowledgeable ex citroen mechanic who loves Xantias for an explaination.
Kenny