It has to be master cylinder.
"It's seized in the morning": the only possible way it can be seized in the morning is if it was seized at night or the rider applied the brake in the morning (common practice for many and should be for all to give the brakes a few pumps to confirm they are fully operational). If the brake seizes and won't release but does when bled then it has to be the master cylinder.
All symptoms described except the impossible "it seized overnight" describe a master cylinder issue.
Sorry, curt & brevity because I'm madly hammering this out on my phone and my thumbs are sore!
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The Smart Money: #1 - ATGATT, #2 - Training (machine skills and survival skills), #3 - The bike; whatever floats yer boat with the money you have left over
