Looks like my first ER9 set is a lemon. After the RD gave me batt drain issues, the right brake lever also has a fault. When pulling the brake lever, at first it travels normally, hits the wall of resistance when the caliper pistons move and brake pads press against the rotor (normal so far). But when more force is applied, a loud clunk comes from the hinge area of the brake lever/shifter, and there is no further force generated by the hydraulics to squeeze harder on the rotor. So essentially, the rear brake doesn't work no matter how hard I pull on the lever. There is felt resistance at the lever, but it feels like something gave way internally of the lever and no fluid is moved from that point. There are no fluid leaks to be seen anywhere.
So to troubleshoot, yet again I had to rob parts from my 2nd ER9 set, installed the right shifter and it totally works fine. No more brake issues.
Contacted 80's designer again to request for a replacement and awaiting their response. Meanwhile I'll try to dissect the lever and see what's wrong if possible.