Rod,
I assume the above-ground check valve is connected to the skimmer line.. If that is true, when you run all skimmer you are sucking air in from the waterfall pump. This is one reason you can't have two pumps sucking from the same source. As a test, glue a cap on the cutoff waterfall line and I bet your skimmer issues go away.
Thanks,
Jim R.
I assume the above-ground check valve is connected to the skimmer line.. If that is true, when you run all skimmer you are sucking air in from the waterfall pump. This is one reason you can't have two pumps sucking from the same source. As a test, glue a cap on the cutoff waterfall line and I bet your skimmer issues go away.
Thanks,
Jim R.