So you are only running a single piston on the rear caliper now? Or did you modify or replace the rear caliper as well?
Just curious.
I have considered drilling or otherwise modifying the calipers to communicate the chambers and decided against it. That's based on two concerns: one is trapping air and the other is structural integrity. Instead, I will use multiple hoses to apply force to both chambers at once. For the rear, that can either be with a a double banjo bolt on the caliper with a jumper line to the other chamber, or a double banjo bolt from the MC to each banjo on the rear. On the front, there will be a double banjo at the MC and either a splitter above the front fender or a double banjo on each caliper with a jumper to the second orifice.
I haven't decided yet whether to connect the master cylinder to the two outers or to use a splitter or jumper or double banjo bolt to serve both chambers. I understand I may need to replace the rear master cylinder, too, but I'm less concerned about that right now. I'll first try connecting only to the two outers since the rear currently serves the SMC and center piston, so probably pushes enough fluid to do that. I doubt I need a three piston rear caliper, but I'm happy to replace the rear MC for higher volume if that's what it takes.
There's a place near me that makes industrial hydraulic hoses and I can get custom braided steel brake lines with any fittings I want, while I wait. So if I change my mind, it's a single trip to the store to get a different hose.