I've gone to 16T sprocket with no problems at all. Did you do it yourself?
There are several things to have in mind while doing it. But they don't matter now as it's been done already. At this point, it seems to me that piston went out of the cylinder while the work has been done. If it went out completely, you lost some oil and air got into the system. If not, you have to squeeze the cylinder between your hands so the piston goes in completely and just then reassemble everything. Still, when I've done mine it didn't go in completely, still needed few millimeters. So, I've tighten the bolts one by one, each one just a bit at the time so the cover would go on evenly.
Since then, I've removed the cylinder twice already to clean up the area around the front sprocket and both times all went well.
As soon as I remove the cylinder, I squeeze it in and put one ziptie around it so the piston wouldn't work its way out. Still, after removing the ziptie I squeeze it in again before bolting it back on.
I think you need to remove everything and start from scratch. As you said you've lost some oil, after the work's done, bleed your clutch system.
