Well I think I tracked it down. About two years ago I changed the housing for my membrane and put new fittings on it. I dug the old one out of a drawer today and guess what? - yup - a backflow valve cleverly disguised as an elbow fitting
And at the same time my holding tank was wearing out but I never bothered to replace it until a couple of months ago. I guess there was never enough back pressure to force it's way back into the membrane until I installed the new one.
Ah well, live and learn............. and spend more money
