This is a common issue. I just did an 80 with the same issues. there are two ports in the master cylinder the bid one in the middle and a VERY small one at the hose end it is under the tab from the plastic reservoir extension. I just used a utility knife and trimmed the tab out of the way so I could get a fine wire (sharpened on a wet stone) into the port to open it. Here is a little trick, you can see the little air bubble in the port. you can use a wire to coax it out, Repeat till you have enough fluid to start the master pumping. There is just as much crap inside the the master cylinder as everywhere else. I got mine clean as I could and got new fluid pumping, kept flushing and the brakes are solid hard now. To get the last bit of air out. with the bike on the side stand and the MC cover bolted back in place turn the bars full left and wait an hour or two, hit the lever once or twice and the last air will bubble back up into the reservoir.
Have to say this;
AFAIK
Once you have used brake parts cleaner the master cylinder HAS to be disassembled and all rubber parts replaced Brake parts cleaner is NOT compatible with the rubber seals they will swell and become useless. That said I tend to spray a little through the hoses to flush them out then flush them with air and fresh fluid before installing. The rebuild kits are kinda pricey, a new master is pretty tempting.