Thanks for your advice about using the hot melt to seal the line joints. Over the last week I have tried sealing the lines as they attach to the carts, pulling ink through the system and trying to print. I then tried sealing at the base where the lines attach to the external tanks. I have sealed all the tanks and all the cartridges and it has made no difference. So it looks like I have a bad CISS. The thing I find hard to understand is why it worked so well for the first 200-300 A3 sheets, they printed no problem when I first bought it. Any other ideas or should I just put it down to experience and go back to normal carts.