I'm probably a month late, but I had this problem and it's the master cylinder. Mine was the pin hole in the bottom of the MC, it was clogged and wouldn't release pressure. Cleaned it and good to go. Oh yeah, I also replaced all hoses, union bolts, rebuilt calipers (new pistons, seals), and rebuilt the MC. All needed to be done, bike hadn't been riden for 10 years. The pinhole was my my problem in the end, I bet it's yours. Change the fluid too, speed bleeders are awesome!
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