JTQs  statement is valid   especially as the various elements are connected and disconnected   without  thought to carefully removing contamination  before connection , so any little bits  can enter the system and stop the various valves/regulators from completely sealing. I serviced a van the last week or two and there were bits of of the chrome/silver  plating  which the plastic  shower heads are coated with  , in the  three way drain down valve!, conclusion :-  at some time the owner must have drained the system and these bits flowed back down the pipes. They were also in the aerators on the end of the taps.  I always turn off the  mains feed to the aquaroll when not in the van  also the pump gets switched off . Never assume an automatic system  will work as expected , things fail, it's the nature of things. Keeps me in a job(part time)