Oh, lame.
How could I possibly read BB5 and see it as juicy 5?
Ok, Round 2 -
A shit cable and a shit lever contribute to spongy cable brakes. Also, a flexy setup, check all the bolts are tight.
I had a front brake that flexed loads, we swapped the cable and it was no better, it was the dud lever.
But you are using a decent lever, so it is likely to be the cable. Which is what everyone else said... Doh.