Front Brakes Issue
I bought a 919 a couple months ago knowing id have to put a little work into it, most of which was cosmetic. The first major issue i noticed was in the front brakes. They seemed to function okay, until youd get riding for a while and the calipers would freeze up and keep the bike completely stopped. it would start with the lever feeling too stiff, then id notice that the brakes were dragging, and after that point i had a about a minute before the bike would stop completely and not budge. I eventually found that the reason for this was that the spring inside of the MC wasn't returning the lever back to its position quickly enough, and sometimes left the MC partially engaged so that there was a slight pressure on the calipers. Anyways it got worse and worse, to the point that you could only ride it for a few minutes before the brakes would lock up. the only way to get them unstuck was to try to coax the piston back out of the MC and relieve the pressure.
I was fortunate to find a used OEM MC for pretty cheap so i ordered it. After that was installed, i bled the brakes (textbook, hassle free brake bleed) and all was well. The lever returned instantly, the lines were nice and solid free of any air, and the brakes worked great. I rode the bike happily for a couple weeks.
One morning I went out to the garage and noticed the lever was pretty squishy, but not enough to cause any actual issues. i kept riding it and by a week later the brakes felt like they needed to be bled again. the lever would go from very soft to completely stiff and back during my rides, but i never lost pressure entirely and the brakes never started dragging. I thought okay, there were probably some bubbles i missed that have made their way down to the calipers, so i set up to bleed the brakes for a second time. I got a hose on the left front calipers bleeder, gave it a few pumps, and opened it up. a very small amount of fluid came out but it was completely free of bubbles. however, the second i cracked the bleeder all pressure in the lever was gone and it went all the way to my grips. i made sure no air was sucked through the bleeder, closed it, and started to pump some more to build pressure and repeat the motion. unfortunately, the pressure never came. the lever continued compressing all the way down to the grip, and when id open the bleeder after a few pumps, only a tiny amount of bubble-less fluid would come out. i tried the other caliper, same thing. no bubbles, barely any fluid release, and still no pressure at all in the lever. I closed both bleeders and pumped the lever probably 200 times and still nothing. i put the MC lid back on and tried a couple hundred more, no luck. What am i missing?