As stated earlier the Military Seaking had a jettison system that exited by the tail wheel. It consisted of two jettison pumps that could be operated individually or at the same time, depending how fast you wanted to get rid of the fuel. As many on here know the fuel system consists of interconnected tanks, so the total “level” would go down with either the Fwd or Aft pump running and the jettison would stop when the pumps lost suction with 200lb (I think) of fuel unjettisonable in each tank. It has been a while so feel free to correct the amount. The jettison gallery runs outside the tanks down the port cabin and behind the thermal barrier out the back. We always made sure to be ready to shut off the jettison valve manually if needed during tests as it would syphon all available fuel until 200lb as stated, if it stuck open.
The Merlin on the other hand has a slightly different concept. It uses the main fuel gallery that is part of the pressure refuel system to facilitate “dumping” of fuel. This is actually quite elegant as it works on the principle that you don’t want to dump fuel at the same time as refuelling so use the same pipework and save space and weight.
There is a jettison pump in each main tank and it also uses the transfer pumps in the transfer tank to empty that one. The tanks empty to 280Kg (appx) and the transfer tank empties completely, with the fuel exiting from the underside of the left sponson.
Cheers