Hydroboost Question

[bfr57]

I found a hydroboost out of a 2005 Tahoe. I opted not to take the MC from donor and get a new one. I've done a HB swap in my hotrod and it works great, so getting this one mach'd up in my '72 for better brake performance. When I saw this one in the Tahoe, it had a return line from the MC reservoir to the HB; it was just a standard rubber line. This line went from the bottom of MC reservoir to the return line on the HB shown in the pic. Got it mounted and then it dawned on me that the brake system is combined with the PS system. What the heck? How does this work? I'm positive that line went from MC to HB. I know I can find another MC reservoir without the nipple and add a return fitting from HB to the PS pump, but just trying to wrap my head about the 2 systems combined?