sounds like air somewhere... with replacing those parts Im surprised you only blead twice. Did the resevoir ever run empty??? If so, you may have sucked air in the opposing end and need to start over.
Be sure to bleed from the farthest from the resevoir to the closest. Start w/the right rear, then left rear then right front then left front. And be sure to check the resevoir twice per wheel that you are bleeding to be sure it does not suck air. In order to flush air from the entire line, you will go through quite a lot of brake fluid and need to fill the res. up at least a few times during the process.
Normally its not that bad, but after replacing parts that use up half the res. to fill (calipers) I can understand your frustration. Personally, I think if you stick it out and start over you'll be happy with the results. And if not, well, unfortunately you'll have to start over again. Its a picky system. Even a little air in there somewhere will ruin the pedal pressure. And remember, the first few inches of the pedal going down is governed by the rear brakes. So if your loosing pedal pressure, there is prob. air in the longer lines somewhere.