Back when I was a Mechanic your situation would not be one, I looked forward to diagnosing with all non-factory style re-engineered aftermarket parts as it always puts more question marks into the equation. That said I completely understand why you did it and this is how I would tackle your situation. First, I would get the car off the ground with the wheels in the air and statically try and reproduce the situation, that means if you have power brakes to also have the engine running and stand on the brakes with the same force you used when it happened. then without shutting off the car (if power brakes) get out and turn each wheel to see if any have drag. If you find drag in one or a pair of wheels crack the bleeder or bleeders and see if this frees them up If they won't free up and are still tight then you are looking for a mechanical and not hydraulic problem. A mechanical problem could range from a stuck pad in its holder to a rear drum brake shoe hanging up because it exceeded its travel limit. I have seen the latter when a self-adjusting brake is not working correctly and over time with shoe wear goes way out of adjustment and causes the edge of the shoe to go past the flats or mounting tabs on the backing plate and then get hung up and only jamming rotational forces can return it. If only one drum is way out of adjustment you wouldn't even notice much in the way of a low pedal.By the way, nice looking car.