Billions of dollars in flood projects have eased fears of levee breaks near California’s capital and some other cities, but state and federal workers are joining farmers with tractors in round-the-clock battles this week to stave off any chain-reaction failure of rural levees protecting farms and farm towns. As the wet winter forces operators of dams to send more water roaring downstream, the struggle to spot and shore up weak spots in nearly 1,600 miles of levees in the Central Valley is unrelenting, said Rex Osborn, spokesman for emergency operations in San Joaquin County, one of the nation’s main farm and dairy counties.