I agree I would not use either gray tank to back flush the black. I have found that if I back flush the black tank with water 4 to 5 times it comes pretty clean. I shut the black tank valve with the black tank sprayer going and leave on for 4 to 5 mins and of course check the monitor and it seems to come pretty clean. I some time get a little leak out of the check valve on the black tank sprayer that is located under the sink in the bedroom. I have at the dealer now and I think it has a bad check valve.
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