Ian,
The only way that would happen is if water in the waste tank is somehow getting past the filter and shutoff valve and travelling down the pipe towards the vacs, usually happens when there is excessive foam in the tank and then the shutoff doesnt work and foam with a bit of water gets down the pipe.
Or maybe you have a loose fitting for the vaccum pipe in the waste tank and water is getting through or a hole in the pipe somewhere ?
Has to be with the waste tank in my mind unless anyone has a better suggestion
Regards
Steve