Keep going. If you have an airlock, it can sometimes be difficult to get it out the bleeder valve. If you can't, check for a kinked line, then perhaps see if the MC is pumping fluid at all. The piston's seal could finally have given up.
DO NOT throw your brake lever away! Send it back to Shimano customer service in So. Cal. I had that problem with a Deore lever and they promptly sent me a new one. Their customer service is nothing less than fantastic. I don't remember what the warranty duration is on XTR though. If you have your receipt, even better.