Several people have problems here, so my 2 cents worth with some suggestions. While vacuum bleeding can work I was never a big fan. You can only draw 30” of Mercury with Vacuum. Working pressure on a hydraulic brake system is probably 1000 PSI, maybe more. Meaning the lines and fittings are probably rated to withstand many times that, like 3000-4000PSI. Meaning brake fluid that can not compress should show any leak better than a vacuum. As far as how many brake pedal pumps to successfully bled a system, it should never take a hundred. Usually anywhere from 1-2 quarts of fluid. I never recirculate the fluid because it could be contaminated. I never use brake fluid from an open container because once open it will absorb moisture.
To start I would have one of the bleeders open on a rear caliper and have a helper slowly pump the brake pedal. At this point no need to close the bleeder screw every time the pedal is down as you are just trying to get fluid to the back of the car. Once the fluid starts spurting, close the bleeder screw when the brake pedal is down, do not release the pedal until the bleeder is closed. Then the helper in the car should pump the pedal several times, while they are holding down the pedal, crack open the screw and tighten bleeder before repeating the process. Eventually the pedal should get higher and harder as all the air is bled out. I usually put a plastic hose on the end of the bleeder screw running to a jar to avoid getting brake fluid on car parts as it will remove paint and makes less of a mess. Cover fender near the master to protect paint from any spilled or splashed fluid. While bleeding the master cylinder cover should be in place to avoid splashing. At the same time the fluid should never get to the bottom of the reservoir or you will induce air into the master.
Assuming you don’t get a firm pedal and it’s not obvious where you have a leak. You can eliminate components such as calipers, lines, fittings, proportioning valve. You can use an inverted flare nut to cap the circuit, weld it shut or try epoxy to create a plug. For male end you could use an inverted flare union. Starting at the rubber hose and T on the rear axel. If that doesn’t give you a firm pedal you could disconnect the line from the rear brake line to the proportioning valve and put the plug in there. Keep in mind when you are putting this plug in you would treat it as a bleeder screw and still have to bleed the circuit. But this should allow you to isolate the problem.
