Working on a 2000 XJ. I've seriously bled the brakes, got new brake fluid throughout! With engine off I can build pressure as normal. With engine running I push the pedal half way down it's 'ok' but any further I lose all pressure and the idiot light comes on. Not leaking fluid from anywhere; pads, shoes, calipers, wh cylinders are all good; even replaced ALL the hardware down to the self adjusters. I'm suspecting the master cylinder may have a 'nick' in one of the seal causing it to lose pressure above a point??????? It's 57 (NEW of course) so I don't wanna just throw money at it! Can I get any confirmation or suggestions