A partial shutdown of 29 U.S. West Coast ports stretched into a third day on Monday ahead of the U.S. labor secretary’s scheduled arrival in San Francisco to try to broker a settlement ending months of disruptions on the cargo-clogged docks. President Barack Obama, under pressure to weigh in on a labor dispute that has rippled through the U.S. commercial supply chain and beyond, said on Saturday he would dispatch Labor Secretary Tom Perez to meet with the two sides in the conflict.