Hydralic pressure being what it is, pretty near constant, I still think the problem is where the pads meet the rotors. I, too, have made the mistake of using too hard of brake pads. What happens is you stomp on the pedal and the pad hits the rotor and just slides around and around the rotor without grabbibg hold of it much. After a few hard stops the rotor and pad will heat up and start to work better. (as in a race track situation) But since we are not running balls to the wall like that the pads and rotor stay mostly cool...and never work as advertised!
Whats it gonna cost? $20.? Try a set of the cheapest you can find at any parts house. That may be your only problem.
Big JIm