Hello - I have googled to find a forum to adress an ongoing problem that professionals haven't been able to fix in 2 attempts. Here is the scenario that needs more ideas so when ever this happens maybe we can find some faster fix.
      Inevitably the water tank up on the hill runs dry about once a year. Usually due to a mechanical problem. But when we get the pump back on line the camp only gets half to a quarter of full pressure. Then we pull our hair out for a while - and sometimes calll in the professionals and after taking apart some pipes and putting them back together without finding problems then at some point the pressure mysteriously comes back. The common elements in the 5 times this has happened is the tank runs dry - we get the water back online - the pressure won't come up - after seemingless pointless work by us and sometimes professionals the pressure comes back up and no one including professionals knows why.
I figure someone out there either knows exactly what the problem is or can give us some new ideas to brain storm on. Thanks for your time! i will be glued to the monitor!

Thanks for responding Scruff.  I am actually trying to help some friend swho have campers arriving today and the tank was empty this morning due to an error - i will describe the system as best i can.  Like you we always keep coming back to something to do with air getting into the system but no one can prove it.  Your idea to have a contingency pipe may have to be done since this seems to happen on a regular basis.  Its an old camp on national forest property so it is a system that probably needs to be overhauled if it wasn't for being on leased land.  The tank is just at 100 feet up (34 Meters) and comes down a 3 inch pipe.  At the base of the hill it immediately splits to a downhill drop of 3 meters on its way to the swimming pool.  The problem has always been assumed to be the area past this junction that continues into the rest of the camp.  In the next 10 meters where it splits 3 ways into 1 inch pipe to the rest of the camp.  This area might have a bow in the pipe from settling.  The rest of the piping goes through uneven terrain and drops further.  But to the kitchen where we don't get any water at all it must climb a meter or 2.  When the pressure is good this is no problem,  thanks for the idea about the sludge!! will go up and take a look at the tank and see how deep it is there.  wouldn't be surprised that refilling the tank would distrub some.  Thanks for your ideas so far!  

A curious problem.. I can't imagine it being an airlock with that amount of head pressure.

Are all the outlets from all 3 1" branches effected equally? Is the tank allowed to fill fully before use? Do you bleed the air from every outlet after filling?

We have been putting all info together and working on it by assuming the behaviour of the system is all about trapped air.  So this morning we drained the camp and then released the torrent from the tank with a outlet open in each building.  We got back most of the pressure even in the camp kitchen.  Then we tried it again but released the torrent from the base of the hill and same results but without an explosion of air coming back up the pipe into the tank.  So if the past behaviour holds up then we should have full pressure within a week or month.  Do you think the rest of the pressure comes back because if there is trapped air then it is slowly absorbed into the pressured water?  Another thing we will try today is releasing the pressure by turning off the valve at the bottom of the hill for a small period and let any air settle and turn pressure back on?  so far acting like its trapped air seems to be working...