I suspect your problem may be due to the lack of a high loop in the intake line. See this on page 14 of the owners manual:
4. Establish a position for the pump
where the pump inlet is no lower
than the bowl top.
5. Bolt or screw the pump in position
using 6mm (¼“) diameter bolts
or screws ensuring that the flow
arrow on the pump is vertically up
or certainly not more than 45° from
IF THE PUMP IS MOUNTED IN
ANY OTHER WAY THE INSTALLATION
WILL BE INEFFICIENT IN
OPERATION AS THE INLET VALVE
CANNOT SEAL EFFECTIVELY.
6. Cut the 38mm (1½“) diameter
outlet hose to the lengths required.
Be sure to allow enough hose from
the top of the pump to take the loop
above the waterline at maximum
heel of the boat.
this is high enough at maximum
angle of roll. Attach the two cut
lengths of hose from the bowl to the
bottom (inlet) of the pump and from
the top of the pump (outlet) to the
7. Connect the 19mm (¾“) bore
inlet hose to the bowl inlet, having
allowed for a loop similar to the outlet
(see point 6) and attach the inlet
hose to the inlet seacock.
I'm unable to find a link to the owners manual (I've asked Blake to provide one), but I do have a PDF in my files that I'll be glad to send you. Stand by for a private message from me.