Brake lever Torx size?

Just got a rearview mirror and I’m trying to remove the brake lever so I can slide the mirror’s mount between the brake reservoir and I can’t seem to find the right size Torx. I’ve got a pretty full set of Torx sockets but nothing I have seems to fit correctly (either that or they’re just so cheap I can’t get it to fit). Anyone? 2015 Sportsman 570 BTW.