Disconnect the (black) ABS Relay in the engine bay fusebox, this prevents the pump pressurising the system. I had mine out when working as the pump running was peeing me off, turn on the ignition, abs light will stay on. But you can feel the pedal better (either by hand or foot, not driving) and see if there's any problems. It would normally indicate a master cylinder problem, because you have no external leaks that does not mean it couldn't be internal. The brake fluid colour is it desert wine colour ? or has it gone black. The pedal pushrod could be wet with fluid and indicting the inner seals has or are starting to go. Check the fluid daily and if you can mark the level using a sharpie, a simple ---- will do, otherwise the MOT testers will think something is a miss. If you are loosing the fluid then, park on a clean piece of land and then using a broom handle and a hole drilled into a piece of wood 12" long, you can jam up a pumped up pedal onto the seat and leave overnight. if the handle as dropped or gone off pressure. You should then see any leaks, I find looking at night, better as the fluid shines. (leave your Suspension up on high, helps looking around)
