I don't think a master cylinder is the cause. Usually if there's a problem with that you will have a low pedal or one that creeps. Start by checking the booster and vacuum to it. Here's a quick check for the booster. With the engine not running pump the pedal a couple of times to bleed off the vacuum. Then with medium pressure on the brake pedal start the engine. You should feel the pedal pressure change as vacuum comes up. Release the pedal, shut the engine off and wait a minute then pump the brake again, as the vacuum bleeds off the pedal should get harder. One big problem you have with a street rod and a vacuum brake booster is the big rumpity rump cams that are so often used will cause low manifold vacuum. The other thing that is often overlooked is the material in the brake pads. The harder metallic pads have a lower frictional coefficient and will require more pedal pressure to stop. Sometimes the pads will get contaminated and create problems. Just a little bit of brake fluid on them will make a big difference in how well it will stop.