If you look at the aft part of your cockpit floor, port side, you can see a stream of water shooting out from the anti-syphon loop when the engine is running. One tiny mod I made was to run a small length of tubing (1/4" maybe) from that anti-syphon loop in the raw water hose to the scupper. I can hear a small gurgle with each start indicating the system has purged the air out and is moving sea water.
Also, if you go below and physically hold the big exhaust hose you can feel it 'shake' as quantities of water shooting through it.
That's about all I know :)
SM 209, Adagio