Zamora had the clearest chance of the first half when Josmar Zambrano connected with a cross from Cesar Gonzalez inside the six-yard box but his shot went over the crossbar.
Flu improved after half-time and scored the only goal courtesy of striker Rafael Sobis with a powerful free-kick in the 79th minute.
Abel Braga's side made it four wins out of four to stay at the top of the pool.
In the same group, Argentinian side Boca Juniors stayed alive in competition by beating compatriots Arsenal 2-0 at home despite playing almost 60 minutes with 10 men.
In the 36th minute, in the middle of an uninspiring first half, Boca midfielder Leandro Somoza saw his second yellow card after a foul on Luciano Leguizamon.
But Boca scored when Pablo Ledesma followed in after Campestrini saved Juan Roman Riquelme's shot and Juan Sanchez Mino added a second in the dying minutes.
Riquelme said: "We won, but we have to improve a lot if we want to be in the Copa Libertadores final."