If it worked great for a week or so and then went bad, I have a hard time thinking that you have a wrong part (as you mentioned earlier). Keep thinking simple.
Usually the hardest things to figure out have the simplest solutions.
Have someone hold the brake pedal down tight and then the secong person goes around and inspects the flexible hoses for bulging first. That one baffled me for a while on one of my vehicles.
You also said you have bled the brakes several time and the same effect. Try taking the lines loose at both ends and blowing compressed air through the lines and valves in the "reverse" direction the fluid flows when pressure is applied. There could be an obstruction that blocks one way but not the reverse.
Replacing parts to find the answer to the problem gets expensive. Know how these things work instead of just how they go together.
Good luck and let us know what you find.
