Kjman
Unfortunately you have not read the sad news we all received earlier today
https://coffeetime.freeflarum.com/d/3542-announcement
For your problem, it is almost certainly the non-returned valve that is not sealing properly.
As already suggested, you would should not need to remove it from the machine to clean it. Do a descale with citric acid. Then purge using the flush lever for 30 seconds. Maybe three or four times
The non-return valve is tiny and the rubber seal Needs the slightest bit of build up to stop it sealing properly.