UPDATE (2/2/2015): My unit failed again (after almost 3 years) and this time I could not rescue it. The little check valve had totally disintegrated. My guess is that there might have been abit of CLR residue left over even though I had thought I rinsed it well enough after cleaning it. That residue ate away at the metal part over time.
This video shows the step by step process that I used to clean the heater core of the Shark Steam Mop Model S3501.
I did not show how to remove the pipe clamps (needed 3 hands to film and remove the clip) - it is straight forward - use the long nosed plier to squeeze together the wing part of the clip and slip it back away from the end of the tube.
I did not have to replace any of my tubing, however if I did I was going to go to the local
automotive supply place and get some high heat tubing (car radiator overflow piping most likely).
Your model may vary, you assume all risks in working on your steam mop. As always, DIY repair will void your warranty.