Just looking at your pics, the master is not original to the 40. It maybe ok. I think in your latest response you say the brakes were softer without it hooked up. I'm guessing you miss spoke. Does the idle change when you depress the brake pedal? You mentioned torn boots on the calipers, have...
If brakes are hard to push it usually is the brake booster. In your pic the green ones vacuum line is not hooked up. A stk 75 40 came with 4 wheel drum brakes.