My hose end fits snug to the freshwater fill and I can build air pressure inside the freshwater tank when filling if I place my finger over the vent hole. You might be able to do this but not letting it pressure go up very much at all is important. With the water pump shut off and all faucets/ valves closed, you might be able to see the leak on the suction side of the water pump then. Be ready with towel to mop up, but I doubt it would be very much water and damage anything anyway.
On another note... it would be best if you used a compressor and set the air pressure very low. it wouldn't take much to get it to bleed water out the leak..
Try removing the cap to the sediment filter and retighten, but look for a good 'O' ring seal too.
