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How deep is the drain?

How tightly is the airline from the compressor to the filter?

Sending the air through the pump plug port on the suction side of the pump is usually easier since the holes is smaller and easier to seal. Better for the filter too.

A 40 PSI blast should, after a few minutes, at least bubble the drains. It won't as much of the water out as I might with my blower since it pushes a lot more air at a lower pressure.

Lower air volume means the air will snake through the pipe's upper half, leaving the lower half still filled with water. High speed, high volume, low pressure air will simply push more of the water in the line out.

Scott