I have a crown head with a concentrate tank that keeps overflowing . I can not see any check valve in the line, from the back of the toliet to the tank, should there be one? IF so were do I find one for this small pickup tubing?

Yes there should be a siphon check valve in that line. You'll see it, how it can be installed, and the part #s in the exploded drawing here: http://www.raritaneng.com/pdf_files/crown_head/L193crownhead v0900.pdf

You can order the parts from Raritan...and btw, it does require replacing from time to time.

Or...you can forget about using the reservoir altogether...just disconnect the hose and replace the bowl elbow with one that doesn't have the little nipple on it. The reservoir is a holdover from the days when waste went directly overboard and no longer serves any useful purpose--not even lubricating the toilet...electric macerating toilets don't need lubricating. And the chemical can react negatively with other holding tank products.