Another 2016 500HO with a brake issue. The brake lever quits working, but the foot pedal keeps working, bled out whole system. Got me working brakes for a few days then lose the lever control again. no leaks, no drips, no torn lines, no sweaty seals to be seen, just seems to be getting air somewhere, started last winter. Must be a flaw of the setup I guess.

[2015 Z Force 800 53/Trail EPS]

Or just a bad master.

NMK maybe correct, the bad part about this system is you might have to accept the educated "guess". The symptom's you described fit perfectly a couple off those I dealt with & yes it was the handle bar MC.

Part of the failure with the 2016 500 HO's was how they where crated from China, the handle bars & handle bar MC were transported in a very low position. Don't remember the exact problem, it did have something to with air trapping the MC being in that low position.

A couple of things to try, strap that brake lever tightly to the bar & leave sit overnight......that will push air thru. The second that works really well if you have the tool. I mightyvac vacuum canister, used primarily for oil changes.....works extremely well for brake bleeding.

https://www.skf.com/mityvac/products/fluid-evacuation-and-dispensing-equipment-and-accessories/fluid-evacuators/mv7300

my ktm 525exc does this all the time . when i`m not going to ride for a while , i squeeze the front brake lever and put a zip tie around it . no more spongy brakes . might work for you