The junior senator from California is making another public push to overhaul GSE refinance rules — this time by calling on the Federal Housing Finance Agency to immediately implement parts of her bill that would allow underwater borrowers to refi with Fannie Mae and Freddie Mac into a lower-rate loan.Last week, Sen. Barbara Boxer, D-CA, sent FHFA Acting Director Edward DeMarco a letter urging the Finance Agency to take action on the parts of S.170 that can be implemented administratively.