I saw the same connection on a boat
last weekend. New water
hose clamped to the cut-off end of the old copper tubing. Same result: Popped off, emptied the water
tank and filled the bilge
. Couldn't hear either pump
(potable water or bilge) while the engine
The quick fix was to "make" a barb on the copper tube with a few wraps of electrical
tape, then clamp beyond that so it couldn't slide off again.
The real fix is to replace the whole thing with marine
water hose or PEX, all the way up to each fixture. If you had to cheat a bit then maybe there's a tubing-to-barb fitting, possibly involving putting a flange on the end of the tubing. Some SharkBite type connectors can work
on copper pipe/tubing, but if it's not a common home water system diameter, finding one to fit correctly may be an issue.