Oddly enough taking it out to a lake and getting in the water might help you find the leak. Otherwise you could stand the boat on its nose and fill the area with water. Last resort is to remove the flap over the speed tube and start over again with soapy water.
With that said, the most likely cause of a leak is at the very rear of the speed tube, or the valve itself.
Check the connection between the tube and transom.
I had a boat which leaked there and was very hard to find the leak...At both sides of the transom are sealings, behind those was the leak...
Worst thing to fix on a Futura is the speed tubes, mainly because of the access issues.
As mentioned above, the valves are often the problem. Change those first. If that doesnt work I have had some success with the liquid PVC like this: Inflatable Boat Sealant Kit 500ml & 1 Litre
Think I will try the valves first, tried the soap and water several times with no joy, filling them up with water with the boat on the nose may be a good idea but a pain to do, unless I take the speed tube out with the boat on the trailer and try and put water in somehow, anyone know the part number or name of the valves on the speed tubes, do they screw out like the semi recessed valves in the tubes? access is a pain but i managed to get them both out and put back in ok, connections I believe are ok.
Scubaqua
