Air blocks, could this be the answer???

[Iron Giant]

@GtrMan  pretty sure we had this the other week if you run your tank dry or near enough empty at the end of a work day, you have the potential of getting a airlock in the system the following working day, I am guessing you are unwittingly creating your own airlock issue. 

I have a non return valve tap outlet on my tank  see pic attached if on my last job of the day their is only a few inches of water left in the tank i will close the outlet on my tank to prevent a airlock on the drive home, but as a rule i never have to close the non return valve tap as i have a decent amount of water left in the van tank at the end of each day.

 

[ks789]

sounds well worth doing, cheers lads.