Sorry Peggie, I seem to have lost you somewhere along the line.
Two topics, duck bill valves and o-rings. Very different things, indeed.

Yes...they are. Dave originally posted a question about an o-ring...the
topic drifted to joker valves.

A "duck bill valve" is a type of valve like a "ball valve", "globe valve",
"check valve", etc.

It's also a rubber one-way valve...if you go here
http://www.sealandtechnology.com/prodrplc.asp and scroll almost to the
bottom of the page, you'll see (though not very well) a pair of 'em.

I'm not personally familiar with "joker valves"...

If you own a boat that has a toilet, you've apparently never done any
maintenance work on it...'cuz every marine toilet has one, and it gets
blamed--unjustly 99% of the time--for just about every toilet problem.
People even replace joker valves in an attempt to solve problems in the
flush water intake!

but my
comments regarding probable causes of leaks when the attaching screws are
"over tightened" is still a viable probability. At least as I visualize a
joker valve...

If you'll go to the drawings at links I posted previously, you won't
have to visualize one...you can see one.

My comments regarding o-ring was in response to a comment of yours that
either said or implied that over tightening screws on an o-ring seal would
cause leakage.

Overtightening screws on a GASKET...I'm not familiar with any
application using o-rings in which screws would be used.

Obviously one should never "over tighten" any screws but the
probable failure in an o-ring seal situation is stripped or broken screws
and not leakage. An o-ring seal will be designed to have the flanges fully
closed when assembled.

What if there are no flanges? Which there wouldn't be in the application
(threaded male cap with round o-ring between it and the female port in
the tank) that Dave asked about. Remember, we're not talking about a
seal that will have any pressure on it--as it would in a pipe
connection--just a cap in a hole in the top of a tank. It only needs to
be sealed sufficiently to prevent gasses from escaping from a
non-pressurize tank.
--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1
http://shop.sailboatowners.com/books...ku=90&cat=1304