This repair was on a pool/spa combo. The section of plumbing that was bad is called the spa make-up. This is where a portion of the water returning back to the pool from the filter is diverted to keep the spa full.

The spa sits above the pool and the check valve that prevents the spa water from backflowing and draining back into the pool when the pump is off was faulty. We replaced the entire spa make-up section to include a new check valve and new flow regulator valve.