First time having manual brakes, are mine bad?
Have someone step on the brake and watch the calipers. Do they expand and "squeeze" the rotors? You can even spread the pads away to make space and watch them push back out when the peddle is applied. I would watch all four do this to try and isolate if you have poor braking because you are only using the back or only the front to stop the car. If two wheels are the only ones functioning right, the brake system will act the way your are describing.
Hence the metering block question again. The post about taking a pressure reading is a great suggestion. If you want to do a visual because you don't have a gauge yet you can at least check to see if you can turn the wheels with the brakes applied. A backyard garage method but simple.
You may have to buy a new metering block. It's the only thing you haven't replaced. If there was a way to bypass the block and go direct that would tell you but a pressure gauge should tell you what you need to know.
I am a bit surprised that a technical support individual who sold the components hasn't troubleshooted the problem for you.......I'm sure you must have googled this?