Insulin 'sucked back' into tubing?

Hi @Colin of Kent I haven't come across that, but am tagging @tim2000s as he may be able to advise.

When I got my 640G 4 years ago, this started happening to me. When I contacted Medtronic I was asked if the pump was in the same room while I was in the shower. When I said it was, I was told it would be the rise in temperature that caused the suck back. I pointed out that I had done the same thing with my previous Medtronic pump and there was no suck back. I was told I must have been lucky, and to try leaving my 640G in another room while in the shower. I did this but it made no difference.

I needed a replacement pump for a different reason and have not had problems with suck back since. Try phoning your pump supplier as it could now be a recognised fault.

I'd agree with @dancer - contact the pump company. That's something that just shouldn't be happening at normal atmospheric pressure.