Over the past 3 years of owning C26 we have had fresh water leaks. These leaks have not been to the extent of your experience emptying half a tank of water. I would suspect the pressure relief valve lifted and did not seat properly until the water pressure decreased because the pump was shut off. In your description of checks you had mentioned " the hose clamp had come lose again". Hose clamps don't really loosen. The clamping force of the hose clamp compresses the hose and after a period of time the hose stays compressed and the hose clamps clamping force "relaxes" but the clamp will still be effective. If the hose clamp is properly sized this connection still should not leak. Reasons hose clamp connections leak, improperly tightened at installation, improper sizing of hose to barb fitting and improper sizing of hose clamps. Over the past three boating seasons I have had intermittent water leaks (fresh water system) (raw water system)( closed cooling system). A quick tightening of the hose clamp seems to resolve the issue. Over tightening is not the answer. This winter I added inspection and tighten all hose clamps in the boat. I found many hose clamps improperly sized for the hose.I also found many hose to barb fitting connections that the barb fitting was not large enough to except double hose clamps but double hose clamps were used.
*Hose clamps are manufactured to operate over a specified diameter range. It is important that the diameter of the hose to be clampled is within this operating range.
*Other specifications to consider when selecting a hose or band clamp include band width and band material.
*Clamps should be installed so that about a quarter-inch of hose is visible between the band and the hose end, and so that the band is completely seated on the barb of the fitting to which the hose is attached. Note that the Coast Guard doesn't require double clamps anywhere on recreational vessels. The American Boat and Yacht Council (ABYC) requires them in only two places — fuel fill hoses and exhaust systems. But both the USCG and ABYC standards are minimums. Below-waterline fittings, including stuffing boxes, should always have two clamps installed if at all possible. Keep in mind that a hose fitting has to be long enough to accommodate two clamps, plus a quarter-inch space between them, otherwise the second one can damage the hose.
I replaced several clamps while inspecting the connections and cut back a few inches of hose when necessary because the hose was damaged from the improperly sized clamp.
The positive note is Fluid Motion is going above the standard of installing double clamps at every connection. The Negative note is the Fluid Motion technician installing the clamps is not properly installing the clamps (sizing, spacing, and confirming the location of the clamp is actually clamping the hose to the fitting).I'm sure fluid Motion has every size clamp available. Quality control! How many post are made "I have a leak"? The answer it's probably a" loose hose clamp"! Before tightening it confirm it is the right clamp and installed properly.