john tomkins fitted an inline non return valve about 2 weeks ago right next to his tank to stop the water coming out of the hose, which allows air in.
hopefully he'll read this and let us know how he got on!
Yes Dave, totally cured it now, bought a non return valve from Dual pumps and kept the 13mm braided hose from tank to pump as low to the van floor as possible.
Was getting air locks 2 to 3 times a day and had to disconnect hose to reel to release air.
This was I think all down to my 400l flat tank when it was 3/4 empty and water sloshing about when driving, introducing air to the system. Never happened on my upright 250l.
As has been said a valve on the tank outlet would do the same thing, if you remember to keep turning it off/on after every job.