One way valve on dishwasher drain line

Anyone know how this valve works and if it can be disassembled? (it is sweated on, so can't just be removed with a wrench)
I have a blockage in the line between the dishwasher and the drain. The impeller does work on the dishwasher so I don't really suspect that there is an olive pip or something in there but SOMETHING is blocking the line somewhere. The water does drain but at quite a slow rate so the dirty water doesn't drain before the clean water is pushed into the cycle and dishes don't get clean any more..
Something seems to be blocking that valve which looks as if it has a large nut on the top. Suspiciously I asked a dishwasher repair man how to clear it and he didn't seem to know what was inside that valve or whether they could be cleaned. .The super in my building says that usually they are replaced by a plumber cutting them out and replacing them. I am wondering why?
He also said that they are only a one way flap and that in theory it should be possible to unblock them.
I cant budge that nut.
Does anyone know if the nut is designed to be taken off or if taking it off will actually reveal the valve for cleaning as it obviously seems to?
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Hugo Ripanykhazov wrote:

The Super is correct. They look like this: http://www.wastewaterdepot.com/valve_brass_swing.html
They're not normally used on dishwashers in single-family residences, but your bldg has probably a need for them or they are locally req'd.
The big nut does give access to the flapper but removal takes *huge* amounts of torque which you probably can't exert in that space without damaging something else.
Cut the sucker out and, if you have to, use 2 compression fittings to put a brand new one in, rather than soldering with a torch in that space.
Some dishwashers have a solenoid valve controlling the drain cycle; if yours is one, the valve could be blocked or defective.
Also, is there a discharge hose from the washer leading into the check valve? If so, you could remove the hose and watch the drainage from there (with some cobbling to prevent spillage).
Jim
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yes, that was how I established that the blockage was between the plastic connector and the place where the half inch brass piping joins the main drain itself (three inches further over from the valve)
Can't I tap a wrench on the nut with a hammer numerous times and/or train a heat gun on the lower part off which it is supposed (not) to come? Am I really being naive in thinking that from the pic to which you referred, it does look suspiciously easy to clean if I can get the top off?

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Hugo Ripanykhazov wrote:

Well, try it; maybe you'll get lucky... Any I've encountered have been a real b***h to get loose. That cap provides a metal-to-metal seal and has to be tight. Jim

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