It's definitely stout stuff, and has a square profile.  Order one rather than sticking something else in there, but you'll have to buy a few other goodies to justify the shipping cost: 
http://www.van-cafe.com/home/van/page_288_118/rear-seat-heater-core-control-valve-o-ring.html 

It will stay flexible and seal for a long time, and sealants won't stick well to the polyethylene surfaces. If this doesn’t work you may need their fancy valve: http://www.van-cafe.com/home/van/page_2193_225/rear_heater_core_control_valve.html 

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Neil N
Sent: Friday, June 05, 2015 3:25 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Searched Re: Rear Heater Valve O Ring,....

Ok. Seems I'm on the right path here.
The heater hose supplied by van-cafe is pretty stout stuff.
Not knocking them, just an observation.

If I can't find the original seal locally, I'll try a generic O ring and thin coat of RTV at flange edge.

Thanks

Neil.

Date:         Sun, 3 Mar 2013 19:33:15 -0500
Reply-To:     Dennis Haynes <d23haynes57@HOTMAIL.COM>

" .... The gasket between the valve and core sometimes is a problem ... A good sealer on the flanges will prevent all trouble. Also, if replacing the hoses use the correct ones. Most American heater hose is too stiff and this will tweak the valve causing to leak at the flange. "

On 6/4/15, Neil N <musomuso@gmail.com> wrote:

> A while ago, while installing my van-cafe heater hose kit, I recall 
> having a tough time slipping the upper hose through the seal at body 
> and onto the valve at rear heater.
....

> My guess is that it was a combination of an old O ring and stress put 
> on that part during, and after, hose install.

--
Neil n

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