Ben 393 Fwd Heads

I have a Ben 393(06) that when the Fwd Fresh water tank gets low I find the Fwd head's hand pump looses sea water (SW) suction on the flushing side.
Is this normal, or do I have a fault with the head hand pump?
When the Fwd Fresh water tank is full the head hand pump works perfectly - surmising that sea water is being forced up the SW inlet pipe of the hand pump by the fact of the bow sitting lower in the water. This then overcomes any difficulty in achieving pump lift.
Any info on the above is most welcome.
Mike