The U.S. economy can’t truly recover until the housing market revives. Yet recent data indicate that prices, already off an average of 30 percent from their peak in 2006, have still not touched bottom. Lending conditions are tight, and mortgage rates are ticking up again. Nearly a quarter of mortgage borrowers are “underwater,” owing more than their houses are worth. Massive federal assistance – $1 trillion in Federal Reserve mortgage-bond purchases; dramatic expansion of Federal Housing Administration (FHA) loans; an Obama administration push to modify existing home loans – has slowed the collapse but not, apparently, ended it.