Not sure how Deano is progressing with his issue, but I have now solved one of mine.
During my recent extended cruise the master cabin head, which is the one day guests mostly use also, started loosing vacuum. Not a huge issue as both heads have 'night switches' to stop the pump from starting up all the time. Apparently the normal cause of leaks is duck bills needing to be replaced, but in this case, at quiet times, I could hear the leak whilst in the head. So, either a loose hose or leaking floor flange. I deferred repairs until back home, then ordered parts.
A few days after the parts arrived I took them down to the boat. After a few more days of procrastinating I disassembled the vacuflush and cleaned everything up. I had to take the base assembly to my workshop at home to remove the completely rusted screw retaining the ball on its shaft. But, with new parts installed I reassembled everything and am now holding vacuum. Great success, one fixed.
Then the front head. A year or so back the bowl would not keep water all that long, so I installed new bowl seals. It is now holding water but still has a slow loss of vacuum. New duck bills, no change. New vacuum switch, no change.
Now for this head I have not been able to hear an air leak, so it may not be the floor flange leaking. If not then it is a hose leak or a perhaps a pinhole in the diaphragm pump. I now have a spare floor flange seal, but right now am not particularly in the mood to have to completely disassemble and clean another head.
Thinking back, I think that this vacuum leak has been there ever since the refit. At that time pumps were rebuilt and PVC pipe run for all but very short hose connections at the head, pump (both sides) and holding tank. The latter hoses connections were easy to check. The one at the head is really only possible to check when replacing the floor flange.
I am wondering whether I might have a very small leak from a join in the PVC pipe run. If so it might be quite tricky to locate and repair as the pipe run is mostly inaccessible. Are there any other sources of vacuum leaks that I should check before starting the floor flange seal replacement process? I am happy to procrastinate for quite a while whilst suggestions come in....