I have had the same problem this week on a proheat, I do not know the model number maybe 1699 or 7900. I have not made the repair, but I have found a couple promising starts. The first website looks like it may have the part, the second website shows how to take the cleaner apart. I am calling my local repair center for the part on Monday. Let me know if you solve this problem. I googled repair bissell proheat.
http://www.marbeck.com/carpetcleaner_parts_bissell.html
http://newzealand.bissell.com/images/IncludeContent/Service_Center_Help/Upright_Deep_Cleaner_Repair_Guide.pdf
Bill, March 2008