I have found that the main valves often begin to leak, probably because they are not made for frequent use indefinitely.
My solution is to cut a hole in the blanking cap, and "engineer" a bib-thingy tap into it, properly sealed. The cap is then screwed back onto the tank, using LOTS of sealant in the threads. to ensure it does not leak. A hose can then be attached to the tap if required.