I/we tried the breathers on the back with a circuit car, would lose about a 1/2 a quart into the catch tank in each 15/20 mile race. Shifted them to the intake side of rocker covers in between pairs of pushrods depending on position of rocker cover bolts. Have done all track cars same way since. No problems. When you think about it and visualise how the
oil stacks up in the rocker covers during acceleration, cornering , braking its the only logical place to fit a breather.